Not A Number (NAN)
Not A Number is a PHP constant, also called NAN. It represents the result of a math function, when the provided arguments are out of the application domain of that function. For example, acos() only works with values between -1 and 1, so processing acos(10) returns a NAN.
This is slightly different from tan($x), with $x = M_PI;. In this case, the returned value is INF.
While a Not A Number value is a float, it is possible to test a number with is_nan(). It is not possible to compare NAN together.
NAN is displayed as NAN.
<?php
$nan = NAN;
var_dump(is_nan($nan)); // true
var_dump(acos(3) === NAN); // false
var_dump(acos(3)); // float(NAN)
?>