预定义常量

下列常量作为 PHP 核心的一部分总是可用的。

**数学常量**
常量名	常量值	描述
M_PI	3.14159265358979323846	Pi
M_E	2.7182818284590452354	e
M_LOG2E	1.4426950408889634074	log_2 e
M_LOG10E	0.43429448190325182765	log_10 e
M_LN2	0.69314718055994530942	log_e 2
M_LN10	2.30258509299404568402	log_e 10
M_PI_2	1.57079632679489661923	pi/2
M_PI_4	0.78539816339744830962	pi/4
M_1_PI	0.31830988618379067154	1/pi
M_2_PI	0.63661977236758134308	2/pi
M_SQRTPI	1.77245385090551602729	sqrt(pi) [5.2.0]
M_2_SQRTPI	1.12837916709551257390	2/sqrt(pi)
M_SQRT2	1.41421356237309504880	sqrt(2)
M_SQRT3	1.73205080756887729352	sqrt(3) [5.2.0]
M_SQRT1_2	0.70710678118654752440	1/sqrt(2)
M_LNPI	1.14472988584940017414	log_e(pi) [5.2.0]
M_EULER	0.57721566490153286061	Euler constant [5.2.0]

只有常量 M_PI 在 PHP 4.0.0 之前就存在，其它所有常量从 PHP 4.0.0 起开始存在。用 [5.2.0] 标记的常量是在 PHP 5.2.0 中增加的。

User Contributed Notes

Hayley Watson 16-Aug-2017 12:11


From PHP 7.2.0, the PHP_FLOAT_* constants are provided to describe the properties of floating point numbers that can be stored; PHP_FLOAT_MAX, for example, is analogous to PHP_INT_MAX and represents the largest possible floating-point number.

steve at websmithery dot co dot uk 12-Jun-2017 11:02


Another observation on INF.



Division by zero results in INF:



<?php

var_dump(1/0); // float(INF)

?>



BUT a 'Division by zero' warning is issued.

pemapmodder1970 at gmail dot com 23-Jun-2016 03:33


Although INF can be used for comparison against normal numbers and as a directed number, and behaves as reciprocal of zero, it is not like limit INF tends to infinity. These operations do not work:



<?php

var_dump(INF / INF); // float(NAN)

var_dump(INF - INF); // float(NAN)

?>



However, it works with arc-tangent:

<?php

var_dump(atan(INF) / M_PI); // float(0.5)

var_dump(atan2(INF, INF) / M_PI); // float(0.25)

var_dump(atan2(1, INF) / M_PI); // float(0)

?>

md2perpe at gmail dot com 29-Sep-2015 02:43


I just learnt of INF today and found out that it can be used in comparisons:



    echo 5000 < INF ? 'yes' : 'no';       // outputs 'yes'

    echo INF < INF ? 'yes' : 'no';        // outputs 'no'

    echo INF <= INF ? 'yes' : 'no';       // outputs 'yes'

    echo INF == INF ? 'yes' : 'no';       // outputs 'yes'



You can also take its negative:



    echo -INF < -5000 ? 'yes' : 'no';    // outputs 'yes'



Division by INF is allowed:



    echo 1/INF;    // outputs '0'

Hayley Watson 22-Mar-2012 12:20


There are also the predefined PHP_INT_MAX and PHP_INT_SIZE constants, that describe the range of possible integer values.