references-in-php.php

<?php

ini_set('memory_limit', '2G');

class ExampleByReference
{
    public function __invoke(&$array)
    {
        foreach ($array as $index => &$value) {
            $this->itemRoutine($value);
        }
    }

    private function itemRoutine(&$item)
    {
         foreach ($item as $index => &$value) {
             $this->itemSubRoutine($value);
         }
    }

    private function itemSubRoutine(&$item)
    {
         foreach ($item as $index => &$value) {
             strlen($value);
         }
    }
}

class ExampleNoReference
{
    public function __invoke($array)
    {
        foreach ($array as $index => $value) {
            $this->itemRoutine($value);
        }
    }

    private function itemRoutine($item)
    {
         foreach ($item as $index => $value) {
             $this->itemSubRoutine($value);
         }
    }

    private function itemSubRoutine($item)
    {
         foreach ($item as $index => $value) {
             strlen($value);
         }
    }
}

function arrayTree($min, $max, $levels = 2)
{
    $result = [];

    if ($levels --) {
        foreach (range($min, $max) as $index) {
            $result[$index] = arrayTree($min, rand(20, 30), $levels);
        }

        return $result;
    }

    $baseString = 'some people think that this string is faster transfered on reference.';
    foreach (range($min, $max) as $index) {
       $result['key_' . $index] = str_shuffle($baseString);
    }

    return $result;
}

$container = new stdClass();

$container->largeArray = arrayTree(1, 10000);

$benchmark = function ($name, $callback, $container) {
    $memory = memory_get_usage();
    $time = microtime(true);

    for ($i = 0; $i < 100; $i ++ ) {
        $callback($container->largeArray);
    }

    $time = microtime(true) - $time;
    $memory = memory_get_usage() - $memory;

    echo "======================\n";
    echo "Benchark Name: $name\n";
    echo "Time: ${time}s\n";
    echo "Peak memory: ${memory}\n";
};

$benchmark('Just Variables', new ExampleNoReference(), $container);
$benchmark('By Reference', new ExampleByReference(), $container);

results.txt

Results for PHP7.1
======================
Benchark Name: Just Variables
Time: 63.145395994186s
Peak memory: 0
======================
Benchark Name: By Reference
Time: 74.283300876617s
Peak memory: 156487344

Results for PHP7.0
======================
Benchark Name: Just Variables
Time: 63.394997119904s
Peak memory: 0
======================
Benchark Name: By Reference
Time: 73.90741610527s
Peak memory: 155995200