There are several ways that you can check that this is the case. One of the simplest ways is to use a dedicated function, is_numeric - just pass it the value and it'll return true or false depending on whether it is a number or not.
Here's an example:
<?php
$formvalue = "3a";
if(is_numeric($formvalue)) { echo "Is a number"; } else { echo "Not a number"; }
?>
This prints 'Not a number' in this instance
