<?php

class DynamicClosure {
    public static function execute(Closure $closure) {
        return call_user_func($closure);
    }
}


echo '<br/>Standard factorial<br/>';

function factorial($n) {
    echo 'call <br/>';
    if ($n == 0) {
        return 1;
    } else {
        return $n * factorial($n - 1);
    }
}

echo factorial(10) . '<br/>'; // called 11 times
echo factorial(11) . '<br/>'; // called 12 times
echo factorial(8)  . '<br/>'; // called 8 times

echo '<br/>Memoized factorial<br/>';

$memoizedFactorial = DynamicClosure::execute(function(){

    // results are cached in an array
    $results = array();

    // the function is passing itself as a bound variable
    return $factorial = function($n) use(&$results, &$factorial) {
        echo 'call <br/>';
        if (!isset($results[$n])) {
            if ($n == 0) {
                $results[$n] = 1;
            } else {
                $results[$n] = $n * $factorial($n - 1);
            }
        }
        return $results[$n];
    };
});

echo $memoizedFactorial(10) . '<br/>'; // called 11 times
echo $memoizedFactorial(11) . '<br/>'; // called only 2 times!
echo $memoizedFactorial(8) . '<br/>';  // called only 1 times!