Predefined Constants

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

**Math constants**
Constant	Value	Description
`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	Euler constant
`PHP_ROUND_HALF_UP`	1	Round halves up
`PHP_ROUND_HALF_DOWN`	2	Round halves down
`PHP_ROUND_HALF_EVEN`	3	Round halves to even numbers
`PHP_ROUND_HALF_ODD`	4	Round halves to odd numbers
`NAN`	NAN (as a float)	Not A Number
`INF`	INF (as a float)	The infinite