hydrate.php

<?php

class Foo
{
    private $foo;
    private $bar;
    private $baz;
}

$hydrator = function(array $data) {
    $this->foo = $data['a'];
    $this->bar = $data['b'];
    $this->baz = $data['c'];
};

$hydrated = new Foo();

$data = array('a' => 1, 'b' => 2, 'c' => 3);
$aHydrator = $hydrator->bindTo($hydrated, $hydrated);
$aHydrator($data);

var_dump($hydrated);

performance.php

<?php
$iterations = 100000;
class A {private $a; private $b; private $c;}

$hydrator = function(array $data) {
    $this->a = $data['a'];
    $this->b = $data['b'];
    $this->c = $data['c'];
};
$data = array('a' => 1, 'b' => 2, 'c' => 3);
$a = new A();
for ($i = 0; $i < $iterations; $i++) {
    $aHydrator = $hydrator->bindTo($a, $a);
    $aHydrator($data);
}

Interesting example.
Small fix: duplication variable $data in hydrate.php

@Spiller, thank you, edited

Thanks, this is an interesting idea.

It seems that Closure::call() might be a better way to do this trick.