### The PI () Function:

The PI () function is used in many mathematical calculations and is approximately equal to 3.14159265358979323846. By default it will use a precision of 14 decimals, however this can be changed and set as high as 20 in the php.ini file. Here are some examples of it in use:

```
<?php
echo pi() ;
echo "<br>";
echo pi () * 4;
?>
```

### Using M_PI:

M_PI is a constant that will produce the same value as PI (). Our previous example could be rewritten using M_PI like this:

```
<?php
echo M_PI;
echo "<br>";
echo M_PI * 4;
?>
```

### Other Variations:

M_PI_2 = Half of PI ()

M_PI_4 = One Quarter PI ()

M_1_PI =One divided by PI ()

M_2_PI = Two divided by PI ()

M_SQRTPI = Square root of PI ()

M_2_SQRTPI = Two Divided by the square root of PI ()