PHP Math functions is_nan We can check a input or variable is a number or not by using is_nan function. Note that we can't check a string with this function.  We can check square root of a negative number with is_nan function. Here is a sample code to check. $v = acos(1.1); if(is_nan($v)){ echo "this is not a number"; }else{ echo "this is a number"; } Out put is this is not a number You can change the line $v=acos(1.1) with different values of variable $v and see the result. $v=5.23; // this is a number $v=4; // this is a number