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          下列常量作为 PHP 核心的一部分总是可用的。

          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。
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          User Contributed Notes 5 notes

          md2perpe at gmail dot com
          3 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'
          Hayley Watson
          7 years ago
          There are also the predefined PHP_INT_MAX and PHP_INT_SIZE constants, that describe the range of possible integer values.
          Hayley Watson
          1 year 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.
          pemapmodder1970 at gmail dot com
          2 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:

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

          However, it works with arc-tangent:
          (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)
          steve at websmithery dot co dot uk
          1 year ago
          Another observation on INF.

          Division by zero results in INF:

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

          BUT a 'Division by zero' warning is issued.


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