Playing with monads, inspired by Crockford's implementation: https://github.com/douglascrockford/monad/blob/master/monad.js

monad.php

<?php

// A silly prototype chain until I can think of something better
class prototype
{
    private $inherited;

    private $instance;

    public function __construct(prototype $prototype = null)
    {
        $this->inherited = $prototype;
        $this->instance = [];
    }

    public function __set($name, $value)
    {
        $this->instance[$name] = $value;
    }

    public function __get($name)
    {
        if (array_key_exists($name, $this->instance)) {
            return $this->instance[$name];
        } else {
            return $this->inherited->$name;
        }
    }

    public function __call($name, $arguments)
    {
        return call_user_func_array($this->$name->bindTo($this), $arguments);
    }
}

// Just a shell around a prototype chain
class monad extends prototype
{
}

// The unit constructor with lift() method
class unit
{
    private $prototype;

    private $modifier;

    public function __construct($modifier)
    {
        $this->modifier = $modifier;

        $this->prototype = new prototype();
    }

    protected function create($value)
    {
        $monad = new monad($this->prototype);

        $monad->bind = function($func, $arguments = array()) use ($value) {
            array_unshift($arguments, $value);
            return call_user_func_array($func, $arguments);
        };

        if ($this->modifier) {
            $modifier = $this->modifier;
            $value = $modifier($monad, $value);
        }

        return $monad;
    }

    public function __invoke($value)
    {
        return $this->create($value);
    }

    public function lift($name, callable $func)
    {
        $unit = $this;

        $this->prototype->$name = function() use ($func, $unit) {
            $result = $this->bind($func, func_get_args());

            return $result instanceof monad ? $result : $unit->create($result);
        };

        return $this;
    }

    public function lift_value($name, $func)
    {
        $this->prototype->$name = function() use ($func) {
            return $this->bind($func, func_get_args());
        };

        return $this;
    }
}

function MONAD(callable $modifier = null)
{
    return new unit($modifier);
}

// LET'S PLAY

function alert($value)
{
    echo "ALERT $value\n";
}

$identity = MONAD();
$monad = $identity('Hello world.');
$monad->bind('alert');

$ajax = MONAD()
    ->lift('alert', 'alert');

$monad = $ajax('Hello ajax.');
$monad->alert();

$maybe = MONAD(function($monad, $value) {
    if (is_null($value)) {
        $monad->bind = function() use ($monad) {
            return $monad;
        };
    }
    return $value;
});

$number = MONAD()
    ->lift('inc', function($value) {
        return $value + 1;
    });

$x = $number(100);

$x->inc()->inc()->bind('alert'); // 102

$monad = $maybe(null);
$monad->bind('alert'); // nothing