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Predefined Constants

The constants below are always available as part of the PHP core.

Math constants
Constant Value Description Availability
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) PHP 5.2.0
M_2_SQRTPI 1.12837916709551257390 2/sqrt(pi)  
M_SQRT2 1.41421356237309504880 sqrt(2)  
M_SQRT3 1.73205080756887729352 sqrt(3) PHP 5.2.0
M_SQRT1_2 0.70710678118654752440 1/sqrt(2)  
M_LNPI 1.14472988584940017414 log_e(pi) PHP 5.2.0
M_EULER 0.57721566490153286061 Euler constant PHP 5.2.0
PHP_ROUND_HALF_UP 1 Round halves up PHP 5.3.0
PHP_ROUND_HALF_DOWN 2 Round halves down PHP 5.3.0
PHP_ROUND_HALF_EVEN 3 Round halves to even numbers PHP 5.3.0
PHP_ROUND_HALF_ODD 4 Round halves to odd numbers PHP 5.3.0
NAN NAN (as a float) Not A Number  
INF INF (as a float) The infinite  

All constants without a labeled PHP version above are available starting with PHP 4.0.0.

PHP Manual