PHP. Руководство по PHP. 2005
is_nan
is_nan
(PHP4>= 4.2.0, PHP5)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
lcg_value