The irrational mathematical constant of Pi can be obtained a few different ways
in PHP. There is a function to give you the value as well as a bunch of
constants that represent Pi as well as fractions of Pi and it’s square root.
Heck, you could even do it the old fashioned way:

```
$pi = 245850922 / 78256779;
```

But that doesn’t make much sense when there’s so many built-ins. To obtain the
value of Pi with PHP’s built in function is easy (and logically named ;):

But why use a function when you could simply reference a constant? I mean, Pi
is a constant already, so for me this makes the most sense:

Then for fractions and square roots of pi we can utilize these:

```
$half_pi = M_PI_2;
$quarter_pi = M_PI_4
$one_over_pi = M_1_PI;
$two_over_pi = M_2_PI;
$sqrt_pi = M_SQRTPI;
$two_over_sqrt_pi = M_2_SQRTPI;
```

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