The constants below are always available as part of the PHP core.
| 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) [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] |
| PHP_ROUND_HALF_UP | 1 | Round halves up [5.3.0] |
| PHP_ROUND_HALF_DOWN | 2 | Round halves down [5.3.0] |
| PHP_ROUND_HALF_EVEN | 3 | Round halves to even numbers [5.3.0] |
| PHP_ROUND_HALF_ODD | 4 | Round halves to odd numbers [5.3.0] |
| NAN | NAN (as a float) | Not A Number |
| INF | INF (as a float) | The infinite |
Only M_PI is available in PHP versions up to and including PHP 4.0.0. All other constants are available starting with PHP 4.0.0. Constants labeled [5.2.0] were added in PHP 5.2.0.