(PHP 4, PHP 5)
pi — Get value of pi
   Returns an approximation of pi.  The returned float
   has a precision based on the
   precision directive in
   php.ini, which defaults to 14.
   Also, you can use the M_PI constant which yields
   identical results to pi().
  
The value of pi as float.
Example #1 pi() example
<?php
echo pi(); // 3.1415926535898
echo M_PI; // 3.1415926535898
?>