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	值	说明
`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)
`M_2_SQRTPI`	1.12837916709551257390	2/sqrt(pi)
`M_SQRT2`	1.41421356237309504880	sqrt(2)
`M_SQRT3`	1.73205080756887729352	sqrt(3)
`M_SQRT1_2`	0.70710678118654752440	1/sqrt(2)
`M_LNPI`	1.14472988584940017414	log_e(pi)
`M_EULER`	0.57721566490153286061	欧拉常数
`PHP_ROUND_HALF_UP`	1	一半的数值时向上取整
`PHP_ROUND_HALF_DOWN`	2	一半的数值时向下取整
`PHP_ROUND_HALF_EVEN`	3	四舍五入为偶数
`PHP_ROUND_HALF_ODD`	4	四舍五入为奇数
`NAN`	NAN（作为浮点数）	非数值
`INF`	INF（作为浮点数）	无穷大

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md2perpe at gmail dot com ¶

9 years ago

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'

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Hayley Watson ¶

12 years ago

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

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pemapmodder1970 at gmail dot com ¶

8 years ago

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)
?>

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Hayley Watson ¶

7 years ago

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.