(PHP 4 >= 4.2.0, PHP 5)
is_nan — Finds whether a value is not a number
$val
   )
   Checks whether val is 'not a number', 
   like the result of acos(1.01).
  
valThe value to check
   Returns TRUE if val is 'not a number',
   else FALSE.
  
Example #1 is_nan() example
<?php
// Invalid calculation, will return a 
// NaN value
$nan = acos(8);
var_dump($nan, is_nan($nan));
?>
The above example will output:
float(NAN) bool(true)