require_once vs autoload

a.php

<?php

function benchmark_using_require_once() {
    require_once 'b.php';
    B::noop();
}

function benchmark_using_autoload() {
    C::noop();
}
spl_autoload_register(function () { include 'c.php'; });

foreach (array(
    'benchmark_using_require_once',
    'benchmark_using_autoload',
) as $f) {
    $s = microtime(true);
    for ($i = 0; $i < 10000; $i++) {
            call_user_func($f);
    }
    $e = microtime(true);
    printf("%s: %.4f sec\n", $f, $e - $s);
}

b.php

<?php

class B {
    public static function noop() { }
}

c.php

<?php

class C {
    public static function noop() { }
}