比较运算符，如同它们名称所暗示的，允许对两个值进行比较。还可以参考 PHP 类型比较表看不同类型相互比较的例子。
| 例子 | 名称 | 结果 |
|---|---|---|
| $a == $b | 等于 | TRUE，如果类型转换后 $a 等于 $b。 |
| $a === $b | 全等 | TRUE，如果 $a 等于 $b，并且它们的类型也相同。 |
| $a != $b | 不等 | TRUE，如果类型转换后 $a 不等于 $b。 |
| $a <> $b | 不等 | TRUE，如果类型转换后 $a 不等于 $b。 |
| $a !== $b | 不全等 | TRUE，如果 $a 不等于 $b，或者它们的类型不同。 |
| $a < $b | 小与 | TRUE，如果 $a 严格小于 $b。 |
| $a > $b | 大于 | TRUE，如果 $a 严格大于 $b。 |
| $a <= $b | 小于等于 | TRUE，如果 $a 小于或者等于 $b。 |
| $a >= $b | 大于等于 | TRUE，如果 $a 大于或者等于 $b。 |
| $a <=> $b | 太空船运算符（组合比较符） | 当$a小于、等于、大于$b时 分别返回一个小于、等于、大于0的integer 值。 PHP7开始提供. |
| $a ?? $b ?? $c | NULL 合并操作符 |
从左往右第一个存在且不为 NULL 的操作数。如果都没有定义且不为 NULL，则返回 NULL。PHP7开始提供。
|
如果比较一个数字和字符串或者比较涉及到数字内容的字符串，则字符串会被转换为数值并且比较按照数值来进行。此规则也适用于 switch 语句。当用 === 或 !== 进行比较时则不进行类型转换，因为此时类型和数值都要比对。
<?php
var_dump(0 == "a"); // 0 == 0 -> true
var_dump("1" == "01"); // 1 == 1 -> true
var_dump("10" == "1e1"); // 10 == 10 -> true
var_dump(100 == "1e2"); // 100 == 100 -> true
switch ("a") {
case 0:
echo "0";
break;
case "a": // never reached because "a" is already matched with 0
echo "a";
break;
}
?>
<?php
// Integers
echo 1 <=> 1; // 0
echo 1 <=> 2; // -1
echo 2 <=> 1; // 1
// Floats
echo 1.5 <=> 1.5; // 0
echo 1.5 <=> 2.5; // -1
echo 2.5 <=> 1.5; // 1
// Strings
echo "a" <=> "a"; // 0
echo "a" <=> "b"; // -1
echo "b" <=> "a"; // 1
echo "a" <=> "aa"; // -1
echo "zz" <=> "aa"; // 1
// Arrays
echo [] <=> []; // 0
echo [1, 2, 3] <=> [1, 2, 3]; // 0
echo [1, 2, 3] <=> []; // 1
echo [1, 2, 3] <=> [1, 2, 1]; // 1
echo [1, 2, 3] <=> [1, 2, 4]; // -1
// Objects
$a = (object) ["a" => "b"];
$b = (object) ["a" => "b"];
echo $a <=> $b; // 0
$a = (object) ["a" => "b"];
$b = (object) ["a" => "c"];
echo $a <=> $b; // -1
$a = (object) ["a" => "c"];
$b = (object) ["a" => "b"];
echo $a <=> $b; // 1
// only values are compared
$a = (object) ["a" => "b"];
$b = (object) ["b" => "b"];
echo $a <=> $b; // 1
?>
对于多种类型，比较运算符根据下表比较（按顺序）。
| 运算数 1 类型 | 运算数 2 类型 | 结果 |
|---|---|---|
| null 或 string | string | 将 NULL 转换为 ""，进行数字或词汇比较 |
| bool 或 null | 任何其它类型 | 转换为 bool，FALSE < TRUE |
| object | object | 内置类可以定义自己的比较，不同类不能比较，相同类和数组同样方式比较属性（PHP 4 中），PHP 5 有其自己的说明 |
| string，resource 或 number | string，resource 或 number | 将字符串和资源转换成数字，按普通数学比较 |
| array | array | 具有较少成员的数组较小，如果运算数 1 中的键不存在于运算数 2 中则数组无法比较，否则挨个值比较（见下例） |
| object | 任何其它类型 | object 总是更大 |
| array | 任何其它类型 | array 总是更大 |
Example #1 标准数组比较代码
<?php
// 数组是用标准比较运算符这样比较的
function standard_array_compare($op1, $op2)
{
if (count($op1) < count($op2)) {
return -1; // $op1 < $op2
} elseif (count($op1) > count($op2)) {
return 1; // $op1 > $op2
}
foreach ($op1 as $key => $val) {
if (!array_key_exists($key, $op2)) {
return null; // uncomparable
} elseif ($val < $op2[$key]) {
return -1;
} elseif ($val > $op2[$key]) {
return 1;
}
}
return 0; // $op1 == $op2
}
?>
Example #2 Transcription of standard array comparison
<?php
// Arrays are compared like this with standard comparison operators
function standard_array_compare($op1, $op2)
{
if (count($op1) < count($op2)) {
return -1; // $op1 < $op2
} elseif (count($op1) > count($op2)) {
return 1; // $op1 > $op2
}
foreach ($op1 as $key => $val) {
if (!array_key_exists($key, $op2)) {
return null; // uncomparable
} elseif ($val < $op2[$key]) {
return -1;
} elseif ($val > $op2[$key]) {
return 1;
}
}
return 0; // $op1 == $op2
}
?>
参见 strcasecmp()，strcmp()，数组运算符和类型章节。
另一个条件运算符是"?:"（或三元）运算符 。
Example #3 赋默认值
<?php
// Example usage for: Ternary Operator
$action = (empty($_POST['action'])) ? 'default' : $_POST['action'];
// The above is identical to this if/else statement
if (empty($_POST['action'])) {
$action = 'default';
} else {
$action = $_POST['action'];
}
?>
TRUE
时的值为 expr2，在
expr1 求值为 FALSE 时的值为
expr3。
自 PHP 5.3 起，可以省略三元运算符中间那部分。表达式
expr1 ?: expr3 在
expr1 求值为 TRUE 时返回
expr1，否则返回
expr3。
Note: 注意三元运算符是个语句，因此其求值不是变量，而是语句的结果。如果想通过引用返回一个变量这点就很重要。在一个通过引用返回的函数中语句 return $var == 42 ? $a : $b; 将不起作用，以后的 PHP 版本会为此发出一条警告。
Note:
建议避免将三元运算符堆积在一起使用。当在一条语句中使用多个三元运算符时会造成 PHP 运算结果不清晰：
Example #4 不清晰的三元运算符行为
<?php
// 乍看起来下面的输出是 'true'
echo (true?'true':false?'t':'f');
// 然而，上面语句的实际输出是't'，因为三元运算符是从左往右计算的
// 下面是与上面等价的语句，但更清晰
echo ((true ? 'true' : 'false') ? 't' : 'f');
// here, you can see that the first expression is evaluated to 'true', which
// in turn evaluates to (bool)true, thus returning the true branch of the
// second ternary expression.
?>
?? this operator does not emit a notice if the left-hand side value does not exist, just like isset()
However, note that
$a = (float) $foo['bar'] ?? 1;
Does emit a warning, as the (float) $foo['bar'] is evaluated first, e.g. it is equivalent to
$a = ( (float) $foo['bar']) ?? 1;
Not to
$a (float) ( $foo['bar'] ?? 1);
A note on null coalescing operators with arithmetic as it may not be obvious for some people.
<?php
// Only $a and $c are defined as:
$a = 4; $c = 2;
// When you do this:
echo $a ?? 0 + $b ?? 0 + $c ?? 0 ; // 4
// You may have expected a result like this:
echo ($a ?? 0)+($b ?? 0)+($c ?? 0); // 6
// But this is basically what's going on:
echo $a ??(0 + $b)??(0 + $c)?? 0 ; // 4
?>
Also note the first and last will produce an E_NOTICE if $a is not defined or null and $b is not defined:
Notice: Undefined variable: b in __FILE__ on line __LINE__
See also:
http://php.net/manual/en/language.operators.precedence.php
?? vs isset() with __isset() and __get()
<?php
class A
{
public function __get($id)
{
return 'a value';
}
}
class B
{
public function __isset($id)
{
return false;
}
public function __get($id)
{
return 'a value';
}
}
class c
{
protected $test = "a value";
public function __isset($id)
{
return true;
}
}
$a = new A();
$b = new B();
$c = new C();
echo isset($a->test) ? $a->test : 'nothing'; // "nothing"
echo $a->test ?? 'nothing'; // "a value", isset() is not called!
echo isset($b->test) ? $b->test : 'nothing'; // "nothing"
echo $b->test ?? 'nothing'; // "nothing", __isset() is called
echo isset($c->test) ? $c->test : 'nothing'; // throw an Exception
echo $c->test ?? 'nothing'; // "nothing"
?>
(and) and (&&) have a slightly different behavior when it comes to comparison.
(&&) and (=) have higher precedence than (and), in the following code:
<?php
$res = true and false;
var_dump($res); //Returns: true
?>
But why? It's because (=) has a higher precedence than (and):
<?php
($res = true) and false;
var_dump($rest); //Returns: true
?>
But not for:
<?php
$bool = true && false;
var_dump($bool); //Returns false
?>
Please note that (or) act the same as (and).
Care must be taken when using the spaceship operator with arrays that do not have the same keys:
- Contrary to the notes above ("Example #2 Transcription of standard array comparison"), it does *not* return null if the left-hand array contains a key that the right-hand array does not.
- Because of this, the result depends on the order you do the comparison in.
For example:
<?php
$a = ['a' => 1, 'b' => 2, 'c' => 3, 'e' => 4];
$b = ['a' => 1, 'b' => 2, 'd' => 3, 'e' => 4];
var_dump($a <=> $b); // int(1) : $a > $b because $a has the 'c' key and $b doesn't.
var_dump($b <=> $a); // int(1) : $b > $a because $b has the 'd' key and $a doesn't.
?>
Do note, using the ternary operator shorthand (since 5.3), omitting the 2nd expression the first expression will only be called once.
Before 5.3 (or not using the shorthand)
<?php
$val = f('x') ? f('x') : false;
// f('x') will be run twice
?>
After 5.3
<?php
$val = f('x') ?: false;
// f('x') will be run once
?>
I wanted to see if two arrays carry the same information even if they are technically different, so I created the following function. Hopefully this is useful to somebody.
Note: This is a different approach than "rshawiii at yahoo dot com" has taken.
<?php
/**
* function to compare the contents of two arrays
*
* This function returns true if all the following are true:
* - arrays' values match
* - arrays' keys match
* - keys-value pairs are the same in both arrays
*
* The order of the key-value pairs does not matter, so even if
* ($array1 == $array2) is technically false, this function might
* still return true like in the following examples:
*
* $a = array(1, 5, 8);
* $b = array(5, 1, 8);
* => true
*
* $a = array('a' => 1, 'b' => 2, 'c' => 'xyz');
* $b = array('a' => 1, 'c' => 'xyz', 'b' => 2);
* => true
*
* $a = array(4 => 1, 7 => 2, 12 => 'xyz');
* $b = array(9 => 2, 3 => 1, 123 => 'xyz');
* => false
*
* @param array
* @param array
* @return bool matching
*/
function same_array_contents($a, $b) {
// easy
if (count($a) != count($b)) {
return false;
}
// complicated 1: different values
$a_values = array_values($a);
$b_values = array_values($b);
sort($a_values);
sort($b_values);
if ($a_values != $b_values) {
return false;
}
// complicated 2: different index names
$a_keys = array_keys($a);
$b_keys = array_keys($b);
sort($a_keys);
sort($b_keys);
if ($a_keys != $b_keys) {
return false;
}
// complicated 3 (same values and same keys): key => value different?
$r = true; // expect match
if (array_keys($a_keys) != $a_keys
|| array_keys($b_keys) != $b_keys
) {
// associative array(s)
foreach ($a_keys as $key) {
if ($a[$key] != $b[$key]) {
$r = false;
}
}
}
return $r;
}
?>
Take note when grouping ternary operations that return either boolean or integer concatenated to a string:
<?php
echo 'hello ' . true ? 1 : 0, //Outputs 1
'hello ' . (true ? 1 : 0); //Outputs hello 1
?>
When the comparison rules for the spaceship operator say that for "object<=>anything" the object is always greater, and for "array<=>anything" the array is always greater.
The rules should be used in the order they are listed. In the latter case, "anything" means "anything except an object".
The former rule says that "object<=>array" will always decide that the object is greater (and evaluate to a positive integer); to be consistent, the comparison "array<=>object" must also always decide that the object is greater (and evaluate to a negative integer).
A < B and still B < A...
$A = [1 => 1, 2 => 0, 3 => 1];
$B = [1 => 1, 3 => 0, 2 => 1];
var_dump($A < $B); // TRUE
var_dump($B < $A); // TRUE
var_dump($A > $B); // TRUE
var_dump($B > $A); // TRUE
Next - C and D are comparable, but neither C < D nor D < C (and still C != D)...
$C = [1 => 1, 2 => 1, 3 => 0];
$D = [1 => 1, 3 => 1, 2 => 0];
var_dump($C < $D); // FALSE
var_dump($D < $C); // FALSE
var_dump($C > $D); // FALSE
var_dump($D > $C); // FALSE
var_dump($D == $C); // FALSE
beware of the fact, that there is no `<==` nor `>==` therefore `false <= 0` will be `true`. php v. 5.4.27
With respect to using the ternary operator as a 'null-coalescing' operator: expr1 ?: expr2, note that expr1 is evaluated only once.
I was interested about the following two uses of the ternary operator (PHP >= 5.3) for using a "default" value if a variable is not set or evaluates to false:
<?php
(isset($some_variable) && $some_variable) ? $some_variable : 'default_value';
$some_variable ?: 'default_value';
?>
The second is more readable, but will throw an ERR_NOTICE is $some_variable is not set. Of course, this could be overcome by suppressing the notice using the @ operator.
Performance-wise, though, comparing 1 million iterations of the three statements
(isset($foo) && $foo) ? $foo : ''
($foo) ?: ''
(@$foo) ?: ''
results in the following:
$foo is NOT SET.
[isset] 0.18222403526306
[?:] 0.57496404647827
[@ ?:] 0.64780592918396
$foo is NULL.
[isset] 0.17995285987854
[?:] 0.15304207801819
[@ ?:] 0.20394206047058
$foo is FALSE.
[isset] 0.19388508796692
[?:] 0.15359902381897
[@ ?:] 0.20741701126099
$foo is TRUE.
[isset] 0.17265486717224
[?:] 0.11773896217346
[@ ?:] 0.16193103790283
In other words, using the long-form ternary operator with isset($some_variable) is preferable overall if $some_variable may not be set.
(error_reporting was set to zero for the benchmark, to avoid printing a million notices...)
Nice and helpful article!)
I would like to ask:
number == null - it converts both types of comparisons to a boolean or numeric type? In the table the author pointed out that to a boolean. But elsewhere I read that to a numeric type.
Be careful with the "==" operator when both operands are strings:
<?php
var_dump('123' == ' 123'); // true
var_dump('1e3' == '1000'); // true
var_dump('+74951112233' == '74951112233'); // true
var_dump('00000020' == '0000000000000000020'); // true
var_dump('0X1D' == '29E0'); // true
var_dump('0xafebac' == '11529132'); // true
var_dump('0xafebac' == '0XAFEBAC'); // true
var_dump('0xeb' == '+235e-0'); // true
var_dump('0.235' == '+.235'); // true
var_dump('0.2e-10' == '2.0E-11'); // true
var_dump('61529519452809720693702583126814' == '61529519452809720000000000000000'); // true in php < 5.4.4
I found a nice way to use of new "?:" operator:
$a = array();
$a['foo'] = 'oof';
$b = @ ($a['foo'] ?: 'No foo');
$c = @ ($a['bar'] ?: 'No bar');
var_dump($b, $c);
Output:
string(3) "oof"
string(6) "No bar"
No error is thrown and $c is set with correct value.
Benefit: no need to use isset.
I came across peculiar outputs while I was attempting to debug a script
<?php
# Setup platform (pre conditions somewhere in a loop)
$index=1;
$tally = array();
# May work with warnings that $tally[$index] is not initialized
# Notice: Undefined offset: 1 in D:\htdocs\colors\ColorCompare\i.php on line #__
# It is an old fashioned way.
# $tally[$index] = $tally[$index] + 1;
# Does not work: Loops to attempt to change $index and values are aways unaffected
$tally[$index] = isset($tally[$index])?$tally[$index]:0+1;
$tally[$index] = isset($tally[$index])?$tally[$index]:0+1;
$tally[$index] = isset($tally[$index])?$tally[$index]:0+1;
/*
# These three lines output:
Array
(
[1] => 1
)
*/
# Works: This is what I need/expect
# $tally[$index] = 1+(isset($tally[$index])?$tally[$index]:0);
print_r($tally);
?>
The second block obviously does not work what one expects.
Third part is good.
it looks that
if you check 0 against a string with == then PHP returns true:
php -r 'var_dump(0 == "statuses");'
-> returns TRUE
but not if your string has a number at the beginning:
php -r 'var_dump(0 == "2statuses");'
-> returns FALSE
from the specs I get it that it attempts a conversion - in this case the string to number.
so better use ===
as always :)
Beware of the consequences of comparing strings to numbers. You can disprove the laws of the universe.
echo ('X' == 0 && 'X' == true && 0 == false) ? 'true == false' : 'sanity prevails';
This will output 'true == false'. This stems from the use of the UNIX function strtod() to convert strings to numbers before comparing. Since 'X' or any other string without a number in it converts to 0 when compared to a number, 0 == 0 && 'X' == true && 0 == false
Please be careful when comparing strings with floats, especally when you are using the , as decimal.
<?php
var_dump($alt);
var_dump($neu);
var_dump($alt == $neu);
?>
string(9) "590217,73"
float(590217,73)
bool(false)
not the float is cast to a string and then string-compared, but the string is cast to a float and then float-compared.
to compare as strings use strval!
<?php
var_dump(strval($alt));
var_dump(strval($neu));
var_dump(strval($alt) == strval($neu));
?>
string(9) "590217,73"
string(9) "590217,73"
bool(true)
In the table "Comparison with Various Types", please move the last line about "Object" to be above the line about "Array", since Object is considered to be greater than Array (tested on 5.3.3)
(Please remove my "Anonymous" post of the same content before. You could check IP to see that I forgot to type my name)
The use of 5.3's shortened ternary operator allows PHP to coalesce a null or empty value to an alternative:
$value = $planA ?: $planB;
My own server doesn't yet run 5.3. A nice alternative is to use the "or" operator:
$value = $planA or $value = planB;
Maybe i am overlooking something but it seems to me that using unset(string) inside a ternary operator creates an error.
(($var1==0 && $var2==0)?unset($var3):$var3=$var1+$var2);
result:
Parse error: syntax error, unexpected T_UNSET
using the traditional form of IF...ELSE works normal.
if($var1==0 && $var2==0) { unset($var3); }
else { $var3=$var1+$var2; }
result:
This unsets var3 or creates a sum of var1+var2 for var3
JP Kleijn
Netherlands
be careful when trying to concatenate the result of a ternary operator to a string
<?php
print '<div>'.(FALSE) ? 'TRUE [bad ternary]' : 'FALSE [bad ternary]';
print '<br><br>';
print '<div>'.((FALSE) ? 'TRUE [good ternary]' : 'FALSE [good ternary]');
?>
yields:
TRUE [bad ternary]
FALSE [good ternary]
this is because the ternary evaluates '<div>'.(FALSE) not (FALSE) - so the end result is TRUE
[Editor's note: consider using ===]
I discover after 10 years of PHP development something awfull : even if you make a string comparison (both are strings), strings are tested like integers and leading "space" character (even \n, \r, \t) is ignored ....
I spent hours because of leading \n in a string ... it hurts my developper sensibility to see two strings beeing compared like integers and not like strings ... I use strcmp now for string comparison ... so stupid ...
Test code :
<?php
test("1234", "1234");
test("1234", " 1234");
test("1234", "\n1234");
test("1234", "1234 ");
test("1234", "1234\n");
function test($v1, $v2) {
echo "<h1>[".show_cr($v1)."] vs [".show_cr($v2)."]</h1>";
echo my_var_dump($v1)."<br />";
echo my_var_dump($v2)."<br />";
if($v1 == $v2) {
echo "EQUAL !";
}
else {
echo "DIFFERENT !";
}
}
function show_cr($var) {
return str_replace("\n", "\\n", $var);
}
function my_var_dump($var) {
ob_start();
var_dump($var);
$dump = show_cr(trim(ob_get_contents()));
ob_end_clean();
return $dump;
}
?>
Displays this ->
[1234] vs [1234]
string(4) "1234"
string(4) "1234"
EQUAL !
[1234] vs [ 1234]
string(4) "1234"
string(5) " 1234"
EQUAL !
[1234] vs [\n1234]
string(4) "1234"
string(5) "\n1234"
EQUAL !
[1234] vs [1234 ]
string(4) "1234"
string(5) "1234 "
DIFFERENT !
[1234] vs [1234\n]
string(4) "1234"
string(5) "1234\n"
DIFFERENT !
I prefer writing (!$a == 'hello') much more than ($a != 'hello'), but I wondered about the performance.
So I did a benchmark:
<?php
for($bench = 0; $bench < 3; $bench++)
{
$start = microtime(true);
$a = 1;
for($i = 0; $i < 100000000; $i++)
{
if(!$a == 'hello') $b++;
}
$end = microtime(true);
echo "Used time: " . ($end-$start) . "\n";
}
?>
and it results with
# if($a != 'hello')
Used time: 12.552895069122
Used time: 12.548940896988
Used time: 12.470285177231
# if(!$a == 'hello')
Used time: 7.6532161235809
Used time: 7.6426539421082
Used time: 7.6452689170837
a function to help settings default values, it returns its own first non-empty argument :
make your own eor combos !
<?php
/*
* Either Or
*
* usage: $foo = eor(test1(),test2(),"default");
* usage: $foo = eor($_GET['foo'], foogen(), $foo, "bar");
*/
function eor() {
$vars = func_get_args();
while (!empty($vars) && empty($defval))
$defval = array_shift($vars);
return $defval;
}
?>
Boolean switch usege
<?php
class User_Exception extends Exception{}
class User{
public function register($login,$pass,$passCheck)
switch(false){
case(strlen($pass) >= 5):
throw new User_Exception("Password must be at last 5 chars length");
case($pass == $passCheck):
throw new User_Exception("Password is not confirmed!");
case(strlen($login) >= 5):
throw new User_Exception("Login must be at last 5 chars length");
//Do other checks
default:
//Do registration
return true;
}
}
//...
}
?>
I couldn't find much info on stacking the new ternary operator, so I ran some tests:
<?php
echo 0 ?: 1 ?: 2 ?: 3; //1
echo 1 ?: 0 ?: 3 ?: 2; //1
echo 2 ?: 1 ?: 0 ?: 3; //2
echo 3 ?: 2 ?: 1 ?: 0; //3
echo 0 ?: 1 ?: 2 ?: 3; //1
echo 0 ?: 0 ?: 2 ?: 3; //2
echo 0 ?: 0 ?: 0 ?: 3; //3
?>
It works just as expected, returning the first non-false value within a group of expressions.
A quick way to do mysql bit comparison in php is to use the special character it stores . e.g
<?php
if ($AvailableRequests['OngoingService'] == '')
echo '<td>Yes</td>';
else
echo '<td>No</td>';
?>
Note: The ternary shortcut currently seems to be of no use in dealing with unexisting keys in an array, as PHP will throw an error. Take the following example.
<?php
$_POST['Unexisting'] = $_POST['Unexisting'] ?: false;
?>
PHP will throw an error that the "Unexisting" key does not exist. The @ operator does not work here to suppress this error.
Most of the time, you may be content with your conditionals evaluating to true if they are evaluating a non-false, non-zero value. You may also like it when they evaluate to false when you use the number 0.
However, there may be times where you want to make a distinction between a non-false value and a boolean true. You may also wish to make a distinction between a boolean false and a zero.
The identity operator can make this distinction for you.
<?php
$a = 'some string';
$b = 123;
$c = 0;
if($a && $b && (!$c))
{ echo "True.\n"; } else { echo "False.\n"; }
if($a == true && $b == true && $c == false)
{ echo "True.\n"; } else { echo "False.\n"; }
if($a === true || $b === true || $c === false)
{ echo "True.\n"; } else { echo "False.\n"; }
?>
The above code outputs the following:
True.
True.
False.
As you can see, in the first two cases, $a and $b are considered true, while $c is considered false. If this wasn't the case, neither of the first two conditionals would have echoed "True."
In the last case, I've cleverly used the || operator to demonstrate that both $a and $b do not evaluate to true with the identity operator, nor does $c evaluate to false.
The === operator can be used to distinguish boolean from non-boolean values.
Note that the "ternary operator" is better described as the "conditional operator". The former name merely notes that it has three arguments without saying anything about what it does. Needless to say, if PHP picked up any more ternary operators, this will be a problem.
"Conditional Operator" is actually descriptive of the semantics, and is the name historically given to it in, e.g., C.
Be careful when using the ternary operator!
The following will not evaluate to the expected result:
<?php
echo "a string that has a " . (true) ? 'true' : 'false' . " condition in. ";
?>
Will print true.
Instead, use this:
<?php
echo "a string that has a " . ((true) ? 'true' : 'false') . " condition in. ";
?>
This will evaluate to the expected result: "a string that has a true condition in. "
I hope this helps.
If you need nested ifs on I var its important to group the if so it works.
Example:
<?php
//Dont Works
//Parse error: parse error, unexpected ':'
$var='<option value="1" '.$status == "1" ? 'selected="selected"' :''.'>Value 1</option>';
//Works:
$var='<option value="1" '.($status == "1" ? 'selected="selected"' :'').'>Value 1</option>';
echo $var;
?>
Note that typecasting will NOT prevent the default behavior for converting two numeric strings to numbers when comparing them.
e.g.:
<?php
if ((string) '0123' == (string) '123')
print 'equals';
else
print 'doesn\'t equal';
?>
Still prints 'equals'
As far as I can tell the only way to avoid this is to use the identity comparison operators (=== and !==).
When you want to know if two arrays contain the same values, regardless of the values' order, you cannot use "==" or "===". In other words:
<?php
(array(1,2) == array(2,1)) === false;
?>
To answer that question, use:
<?php
function array_equal($a, $b) {
return (is_array($a) && is_array($b) && array_diff($a, $b) === array_diff($b, $a));
}
?>
A related, but more strict problem, is if you need to ensure that two arrays contain the same key=>value pairs, regardless of the order of the pairs. In that case, use:
<?php
function array_identical($a, $b) {
return (is_array($a) && is_array($b) && array_diff_assoc($a, $b) === array_diff_assoc($b, $a));
}
?>
Example:
<?php
$a = array (2, 1);
$b = array (1, 2);
// true === array_equal($a, $b);
// false === array_identical($a, $b);
$a = array ('a' => 2, 'b' => 1);
$b = array ('b' => 1, 'a' => 2);
// true === array_identical($a, $b)
// true === array_equal($a, $b)
?>
(See also the solution "rshawiii at yahoo dot com" posted)
I think everybody should read carefully what "jeronimo at DELETE_THIS dot transartmedia dot com" wrote. It's a great pitfall even for seasoned programmers and should be looked upon with a great attention.
For example, comparing passwords with == may result in a very large security hole.
I would add some more to it:
The workaround is to use strcmp() or ===.
Note on ===:
While the php documentation says that, basically,
($a===$b) is the same as ($a==$b && gettype($a) == gettype($b)),
this is not true.
The difference between == and === is that === never does any type conversion. So, while, according to documentation, ("+0.1" === ".1") should return true (because both are strings and == returns true), === actually returns false (which is good).
Note: according to the spec, PHP's comparison operators are not transitive. For example, the following are all true in PHP5:
"11" < "a" < 2 < "11"
As a result, the outcome of sorting an array depends on the order the elements appear in the pre-sort array. The following code will dump out two arrays with *different* orderings:
<?php
$a = array(2, "a", "11", 2);
$b = array(2, "11", "a", 2);
sort($a);
var_dump($a);
sort($b);
var_dump($b);
?>
This is not a bug report -- given the spec on this documentation page, what PHP does is "correct". But that may not be what was intended...
You can't just compare two arrays with the === operator
like you would think to find out if they are equal or not. This is more complicated when you have multi-dimensional arrays. Here is a recursive comparison function.
<?php
/**
* Compares two arrays to see if they contain the same values. Returns TRUE or FALSE.
* usefull for determining if a record or block of data was modified (perhaps by user input)
* prior to setting a "date_last_updated" or skipping updating the db in the case of no change.
*
* @param array $a1
* @param array $a2
* @return boolean
*/
function array_compare_recursive($a1, $a2)
{
if (!(is_array($a1) and (is_array($a2)))) { return FALSE;}
if (!count($a1) == count($a2))
{
return FALSE; // arrays don't have same number of entries
}
foreach ($a1 as $key => $val)
{
if (!array_key_exists($key, $a2))
{return FALSE; // uncomparable array keys don't match
}
elseif (is_array($val) and is_array($a2[$key])) // if both entries are arrays then compare recursive
{if (!array_compare_recursive($val,$a2[$key])) return FALSE;
}
elseif (!($val === $a2[$key])) // compare entries must be of same type.
{return FALSE;
}
}
return TRUE; // $a1 === $a2
}
?>
The following contrasts the trinary operator associativity in PHP and Java. The first test would work as expected in Java (evaluates left-to-right, associates right-to-left, like if stmnt), the second in PHP (evaluates and associates left-to-right)
<?php
echo "\n\n######----------- trinary operator associativity\n\n";
function trinaryTest($foo){
$bar = $foo > 20
? "greater than 20"
: $foo > 10
? "greater than 10"
: $foo > 5
? "greater than 5"
: "not worthy of consideration";
echo $foo." => ".$bar."\n";
}
echo "----trinaryTest\n\n";
trinaryTest(21);
trinaryTest(11);
trinaryTest(6);
trinaryTest(4);
function trinaryTestParens($foo){
$bar = $foo > 20
? "greater than 20"
: ($foo > 10
? "greater than 10"
: ($foo > 5
? "greater than 5"
: "not worthy of consideration"));
echo $foo." => ".$bar."\n";
}
echo "----trinaryTestParens\n\n";
trinaryTestParens(21);
trinaryTestParens(11);
trinaryTest(6);
trinaryTestParens(4);
?>
Output:
######----------- trinary operator associativity
----trinaryTest
21 => greater than 5
11 => greater than 5
6 => greater than 5
4 => not worthy of consideration
----trinaryTestParens
21 => greater than 20
11 => greater than 10
6 => greater than 5
4 => not worthy of consideration
if you want to use the ?: operator, you should be careful with the precedence.
Here's an example of the priority of operators:
<?php
echo 'Hello, ' . isset($i) ? 'my friend: ' . $username . ', how are you doing?' : 'my guest, ' . $guestusername . ', please register';
?>
This make "'Hello, ' . isset($i)" the sentence to evaluate. So, if you think to mix more sentences with the ?: operator, please use always parentheses to force the proper evaluation of the sentence.
<?php
echo 'Hello, ' . (isset($i) ? 'my friend: ' . $username . ', how are you doing?' : 'my guest, ' . $guestusername . ', please register');
?>
for general rule, if you mix ?: with other sentences, always close it with parentheses.
For converted Perl programmers: use strict comparison operators (===, !==) in place of string comparison operators (eq, ne). Don't use the simple equality operators (==, !=), because ($a == $b) will return TRUE in many situations where ($a eq $b) would return FALSE.
For instance...
"mary" == "fred" is FALSE, but
"+010" == "10.0" is TRUE (!)
In the following examples, none of the strings being compared are identical, but because PHP *can* evaluate them as numbers, it does so, and therefore finds them equal...
<?php
echo ("007" == "7" ? "EQUAL" : "not equal");
// Prints: EQUAL
// Surrounding the strings with single quotes (') instead of double
// quotes (") to ensure the contents aren't evaluated, and forcing
// string types has no effect.
echo ( (string)'0001' == (string)'+1.' ? "EQUAL" : "not equal");
// Prints: EQUAL
// Including non-digit characters (like leading spaces, "e", the plus
// or minus sign, period, ...) can still result in this behavior, if
// a string happens to be valid scientific notation.
echo (' 131e-2' == '001.3100' ? "EQUAL" : "not equal");
// Prints: EQUAL
?>
If you're comparing passwords (or anything else for which "near" precision isn't good enough) this confusion could be detrimental. Stick with strict comparisons...
<?php
// Same examples as above, using === instead of ==
echo ("007" === "7" ? "EQUAL" : "not equal");
// Prints: not equal
echo ( (string)'0001' === (string)'+1.' ? "EQUAL" : "not equal");
// Prints: not equal
echo (' 131e-2' === '001.3100' ? "EQUAL" : "not equal");
// Prints: not equal
?>
With Nested ternary Operators you have to set the logical parentheses to get the correct result.
<?php
$test=true;
$test2=true;
($test) ? "TEST1 true" : ($test2) ? "TEST2 true" : "false";
?>
This will output: TEST2 true;
correct:
<?php
$test=true;
$test2=true;
($test) ? "TEST1 true" : (($test2) ? "TEST2 true" : "false");
?>
Anyway don't nest them to much....!!
note: the behavior below is documented in the appendix K about type comparisons, but since it is somewhat buried i thought i should raise it here for people since it threw me for a loop until i figured it out completely.
just to clarify a tricky point about the == comparison operator when dealing with strings and numbers:
('some string' == 0) returns TRUE
however, ('123' == 0) returns FALSE
also note that ((int) 'some string') returns 0
and ((int) '123') returns 123
the behavior makes senes but you must be careful when comparing strings to numbers, e.g. when you're comparing a request variable which you expect to be numeric. its easy to fall into the trap of:
if ($_GET['myvar']==0) dosomething();
as this will dosomething() even when $_GET['myvar'] is 'some string' and clearly not the value 0
i was getting lazy with my types since php vars are so flexible, so be warned to pay attention to the details...