Memory efficient array permutation in PHP 5.5 using generators

Generators support was introduced in PHP 5.5.
The yield keyword is a really cool thing that allows to write an elegant and memory efficient code.
It was existing in another dynamic languages like python for quite a long time and now it is in PHP as well!
To demonstrate generators concept, i wrote a sample function that yields all permutations of the given array and avoids of data copying on recursive calls:

function permute($array) { 
    $length = sizeof($array); 
    $inner = function($ix = []) use ($array, $length, &$inner) { 
        $yield = sizeof($ix) == $length - 1; 
        for ($i = 0; $i < $length; $i++) { 
            if (in_array($i, $ix)) { 
                continue; 
            } elseif ($yield) { 
                $toYield = []; 
                foreach (array_merge($ix, [$i]) as $index) { 
                    $toYield[] = $array[$index]; 
                } 
                yield $toYield; 
            } else { 
                foreach ($inner(array_merge($ix, [$i])) as $permute) { 
                    yield $permute; 
                } 
            } 
        } 
    }; 
    return $inner(); 
} 

Usage:

foreach (permute([1, 2, 3]) as $permutation) {
    echo join(' ', $permutation), "\n";
}

Output:

1 2 3
1 3 2
2 1 3
2 3 1
3 1 2
3 2 1

Memory usage is staying almost same regardless of the array size:

foreach (permute(range(0, 3)) as $permutation) {
    //echo join(' ', $permutation), "\n";
}
echo memory_get_peak_usage() / 1024; //in kb

Output: 265.3359375


foreach (permute(range(0, 8)) as $permutation) {
    //echo join(' ', $permutation), "\n";
}
echo memory_get_peak_usage() / 1024; //in kb

Output: 292.59375