you could use ajax to call a php validation function on the server side, then also check the values when the form is submitted using the same php function. this is the method I tend to use if I want any client side validation, though you said no javascript, as it means my validation code is only in one place and isnt duplicated in two places. Though it does mean more traffic between the client and the server.
you can use is_numeric or ctype_isdigit or a regular expression