There are several values in mathematics that are used in maths-related scripts but take some time to calculate, so, to save time, PHP defines them as constants available to you in every script. For example, if you want to use the value of Pi, you can just use the constant value M_PI.

For example, to calculate the area *a* of a circle based upon its radius *r* , the formula is *a* = pi * *r* 2. Using PHP we can write this as:

```
<?php
$area = M_PI * ($radius * $radius);
// or...
$area = M_PI * pow($radius, 2);
?>
```

Using PHP's built-in maths constants saves a lot of lengthy (and pointless) calculation, so keep in mind what you have got available to you if you ever need to use mathematical constants. Here is a list of the most popular constants:

Constant |
Value |
Meaning |

M_PI |
3.14159265358979323846 |
Pi |

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(M_PI) |

M_2_SQRTPI |
1.12837916709551257390 |
2/sqrt(M_PI) |

M_SQRT2 |
1.41421356237309504880 |
sqrt(2) |

M_SQRT3 |
1.73205080756887729352 |
sqrt(3) |

M_SQRT1_2 |
0.70710678118654752440 |
1/sqrt(2) |

**Next chapter: Playing with strings >>**

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