Apparently NAN (Not A Number) is a number for the sake of is_numeric(). 
<?php 
echo "is "; 
if (!is_numeric(NAN)) 
 echo "not "; 
 echo "a number"; 
?> 
Outputs "is a number". So something that is NOT a number (by defintion) is a number...Apparently NAN (Not A Number) is a number for the sake of is_numeric(). 
<?php 
echo "is "; 
if (!is_numeric(NAN)) 
 echo "not "; 
 echo "a number"; 
?> 
Outputs "is a number". So something that is NOT a number (by defintion) is a number...