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 5.0.0.

User Contributed Notes

md2perpe at gmail dot com

7 months 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

4 years ago


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