addMethodAtRuntime.php

<?php

class T
{
    protected $methods = array();

    public function addMethod($name, $closure) {
        $this->methods[$name] = $closure->bindTo($this, __CLASS__);
    }

    function __call($method, $args) {
        if (is_callable($this->methods[$method])) {
            return call_user_func_array($this->methods[$method], $args);
        }
    }
}

$t = new T;
$t->addMethod("hello", function($name) {
    echo "Hello $name !";
});

$t->hello("Seb");