All this talk of functional language features is great, but Lithium is still in development, Zend Framework 2.0 is a ways away, you have an upcoming project deadline and you want to know how this stuff can help you now.  Easy: go read or watch JavaScript examples.  Seriously.  Watching Douglas Crockford's videos on YUI theatre or reading through ExtJS source code can give you really good insight into the power of functional language features.  The syntax may be different, but the core ideas are the same.

The new functional feel of PHP seems to have started a trend towards a unified constructor.  This has been a common practice in the JavaScript community for some time.  The great thing is that the PHP array is very similar to the JavaScript object.  In fact, the JSON extension to PHP makes them completely interchangeable.  Here is a quick example of one unified constructor implementation in PHP:

<?php
 
$p = array(
    'a' => 'hi',
    'adder' => function($n) { return $n + 1; },
    'c' => 3,
    'd' => new stdClass
);
 
class base {
    function __construct(array $params)
    {
        foreach ($params as $key => $value) {
            $this->$key = $value;
        }
    }
}
 
class foo extends base { }
 
$foo = new foo($p);

I have a bunch of ideas (some great, some maybe not so great) that I plan on implementing using lambda functions and closures. I will be adding new components to my Crimson library and discussing them here.