Is Not A NAN

<?php
  
var_dump(NAN === NAN); // false

var_dump(is_nan(NAN)); // true
  
?>

NAN is a special PHP constant, which represents not-a-number: it is returned when a math function cannot calculate a value because it is out of its domain of application. For example, acos(3) returns NAN, because cos() produces values between -1 and 1.

One special situation with NAN is that it is always different from anything. So, it cannot even compare to itself.

Also, NAN is a float.

See Also

• NAN (PHP manual)
• is_nan (PHP manual)
• NAN !== NAN [Try me]

PHP Features

• nan
• php-constant
• float

Last updated: 10 September 2026