memoize_example.php

<?php

$result = function_that_does_crazy_stuff($arg1, $arg2);
// becomes
$result = memoize('function_that_does_crazy_stuff', $arg1, $arg2);

$result = StaticClass::method($arg1, $arg2);
// becomes
$result = memoize(array('StaticClass', 'method'), $arg1, $arg2);

$object = new SomeClass;
$result = $object->method($arg1, $arg2);
// becomes
$result = memoize(array($object, 'method'), $arg1, $arg2);

you might want to initialize your variables up at the top. Empty functions would probably be the logical choice. Maintaining type consistency is key. Certainly stay away from arrays.