Petah

<?php
// Initialise input
define('TEST_SIZE', !isset($_GET['test_size']) || !is_numeric($_GET['test_size']) ? 10 : $_GET['test_size']);
define('TYPE', !isset($_GET['type']) || !in_array($_GET['type'], array('array_walk', 'foreach')) ? 'array_walk' : $_GET['type']);
define('TRUE_USAGE', isset($_GET['true_usage']));

// Create form for test input
?>
<form action="" method="get">
    <fieldset>
        <input type="checkbox" name="true_usage" <?php if (TRUE_USAGE) echo 'checked'; ?>>
        <label>True Memory Usage</label>
    </fieldset>

    <fieldset>
        <label>Test Size</label>
        <input name="test_size" type="text" value="<?php echo TEST_SIZE; ?>">
    </fieldset>

    <fieldset>
        <label>Type</label><br>
        <input name="type" type="radio" value="array_walk" <?php if (TYPE == 'array_walk') echo 'checked'; ?>>
        <label>array_walk</label><br>
        <input name="type" type="radio" value="foreach" <?php if (TYPE == 'foreach') echo 'checked'; ?>>
        <label>foreach</label>
    </fieldset>

    <button type="submit">Run</button>
</form>

<pre>
Benchmark

<?php
// Create a class for testing
class Test {
    private static $memory = null;
    private static $memory_peak = null;
    private static $time = null;

    public static function start() {
        self::$memory = memory_get_usage(TRUE_USAGE);
        self::$memory_peak = memory_get_peak_usage(TRUE_USAGE);
        self::$time = microtime(true);
        echo "Test started: ".TYPE.PHP_EOL;
    }

    public static function stop() {
        echo "Test stopped".PHP_EOL;
        echo "Memory: \t".number_format(memory_get_usage(TRUE_USAGE) - self::$memory).PHP_EOL;
        echo "Peak memory: \t".number_format(memory_get_peak_usage(TRUE_USAGE) - self::$memory_peak).PHP_EOL;
        echo "Time: \t\t".number_format(microtime(true) - self::$time, 6).PHP_EOL;
    }
}

// Initialise the data
$recordset = array();
for ($i = 0; $i++ < TEST_SIZE;) {
    $recordset[] = array('code' => $i, 'quantity' => rand(10, 30), 'amount' => rand(1, 5));
}

$total = array('quantity' => 0, 'amount' => 0, 'price' => null);

// Run the test
switch (TYPE) {
    case 'array_walk':
        Test::start();
        array_walk($recordset, function (&$value) use(&$total) {
            $value['price'] = ($value['amount'] == 0) ? null : $value['quantity'] / $value['amount'];
            $total['quantity'] += $value['quantity'];
            $total['amount'] += $value['amount'];
        });
        Test::stop();
        break;
    case 'foreach':
        Test::start();
        foreach ($recordset as $value) {
            $value['price'] = ($value['amount'] == 0) ? null : $value['quantity'] / $value['amount'];
            $total['quantity'] += $value['quantity'];
            $total['amount'] += $value['amount'];
        }
        Test::stop();
        break;
}

// Get the result
$total['price'] = ($total['amount'] == 0) ? null : $total['quantity'] / $total['amount'];
var_dump($total);
?>
</pre>