(PHP 4, PHP 5)
mt_getrandmax — Show largest possible random value
Returns the maximum value that can be returned by a call to mt_rand().
   Returns the maximum random value returned by a call to
   mt_rand() without arguments, which is the maximum value 
   that can be used for its max parameter without the
   result being scaled up (and therefore less random).
  
Example #1 Calculate a random floating-point number
<?php
function randomFloat($min = 0, $max = 1) {
    return $min + mt_rand() / mt_getrandmax() * ($max - $min);
}
var_dump(randomFloat());
var_dump(randomFloat(2, 20));
?>
The above example will output something similar to:
float(0.91601131712832) float(16.511210331931)