<?php
class Foo {
    public function __call($methodName, $parameters) {
        if (strpos($methodName, 'getValid') !== false) {
            $propName = lcfirst(substr($methodName, 8));
            if (property_exists($this, $propName)) {
                if (property_exists($this, $methodName)) {
                    $anoMeth = $this->$methodName;
                    return $anoMeth($this);
                } else {
                    return htmlspecialchars($this->$propName);
                }
            } else {
                return null;
            }
        }
    }
}

$foo = new Foo();

$foo->name = 'blubber';
$foo->getValidName = function($obj) {
    return htmlspecialchars($obj->name);
};

$foo->number = 13;
echo $foo->getValidNumber() . '<br />';
echo $foo->getValidName();