You can use ctype_digit() to check for numeric character(s)
http://www.php.net/manual/en/function.ctype-digit.php
<?php
$numeric_string = '42';
$integer = 42;
ctype_digit($numeric_string); // true
ctype_digit($integer); // false (ASCII 42 is the * character)
is_numeric($numeric_string); // true
is_numeric($integer); // true
?>