Methods for any properties that require user input should always be the same name as the property key you have specified when adding a product. So, assume you have added
      a product with two properties and they both require user input.
      So, here the methods should be named 'Param' & 'Key'. Example:
      
function Param($input) {
      }
      function Key($input) {
      }
      Inside these functions is your validation. So, lets say that the value of 'Key' can only be numeric, you might do:
      
function Key($input) {
        return (ctype_digit($input) ? true : false);
      }
      This uses the ctype function. You might also do:
      
function Key($input) {
        if (is_numeric($input)) {
          return true;
        } else {
          return false;
        }
      }
      Regular expressions can also be used:
      
function Key($input) {
        if (preg_match('/^\d+$/', $input)) {
          return true;
        } else {
          return false;
        }
      }
      Prefix 'cube_' to property keys that begin with a number. So, lets say your property key is '123key', for input validation use the following function name:
      
function cube_123key($input) {
       xxxx
      }
      More examples below.