Here is an overview of the mathematical constants in PHP

Constant Exact value Description
M_PI                    3.14159265358979323846          The value of (Pi)
M_E                      2.7182818284590452354             E (Euler number)
M_LOG2E           1.4426950408889634074            log2 e
M_LOG10E        0.43429448190325182765          log10 e
M_LN2                0.69314718055994530942          loge 2
M_LN10              2.30258509299404568402        loge 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_2_SQRTPI    1.12837916709551257390         2/sqrt (pi)
M_SQRT2           1.41421356237309504880          sqrt (2)
M_SQRT1_2      0.70710678118654752440        1/sqrt (2)

Until M_PI constants are only available from PHP 4.0.