php code for perfomance testing of your code

test.php

// Test results
$array1 = test('array_walk');
$array2 = test('array_walk_list_each');
$array3 = test('array_walk_foreach1');
$array4 = test('array_walk_foreach2');

// Check arrays for equal

var_dump($array1 == $array2, $array1 == $array3, $array1 == $array4);

// Test function 1
function array_walk_list_each(&$array, $function, $userData = null) {
    while ( list($key, $value) = each($array) )
        $function($array[$key], $key, $userData);
}

// Test function 2
function array_walk_foreach1(&$array, $function, $userData = null) {
    foreach ($array as $key => &$value )
        $function($value, $key, $userData);
}

// Test function 3
function array_walk_foreach2(&$array, $function, $userData = null) {
    foreach ($array as $key => $value )
        $function($array[$key], $key, $userData);
}

function some_function(&$value, $key, $userData) {
    $value = "$key => $userData";
}

function test($function, $count = 10000, $arrayElements = 1000) {
    echo $function, ' ... ';
    $array = array_fill(0, $arrayElements, "some text value");
    
    $timer = microtime(true);
    for( $i = 0; ++$i < $count; )
        $function($array, 'some_function', 'some user data');
    printf("%.3f sec <hr>", microtime(true) - $timer);
    
    return $array;
}