PHP Manual - collected by Jimbob 2006.03
is_nan
(PHP 4 >= 4.2.0, PHP 5)
is_nan -- Finds whether a value is not a number
Description
bool
is_nan ( float val )
Returns TRUE if val is 'not a number',
like the result of acos(1.01).
See also is_finite() and
is_infinite().