问为什么"hash('md5'，'string')“比"md5('string')”快？

EN

set_time_limit(0);
echo "<pre>";

function m1($total) {
    for($i = 0; $i < $total; $i ++)
        hash('md5', 'string');
}

function m2($total) {
    for($i = 0; $i < $total; $i ++)
        md5('string');
}

function m3($total) {
    for($i = 0; $i < $total; $i ++)
        hash('sha1', 'string');
}

function m4($total) {
    for($i = 0; $i < $total; $i ++)
        sha1('string');
}

function m5($total) {
    for($i = 0; $i < $total; $i ++)
        hash('md5', $i);
}

function m6($total) {
    for($i = 0; $i < $total; $i ++)
        md5($i);
}

function m7($total) {
    for($i = 0; $i < $total; $i ++)
        hash('sha1', $i);
}

function m8($total) {
    for($i = 0; $i < $total; $i ++)
        sha1($i);
}

$result = array(
        'm1' => 0,
        'm2' => 0,
        'm3' => 0,
        'm4' => 0,
        'm5' => 0,
        'm6' => 0,
        'm7' => 0,
        'm8' => 0
);

$total = 10000;

for($i = 0; $i < 100; ++ $i) {
    foreach ( array_keys($result) as $key ) {
        $alpha = microtime(true);
        $key($total);
        $result[$key] += microtime(true) - $alpha;
    }
}

echo '<pre>';
echo "Single Run\n";
print_r($result);
echo '</pre>';

Single Run
Array
(
    [m1] => 0.58715152740479                 <--- hash/md5/string
    [m2] => 0.41520881652832                 <--- md5/string
    [m3] => 0.79592990875244                 <--- hash/sha1/string
    [m4] => 0.61766123771667                 <--- sha1/string
    [m5] => 0.67594528198242                 <--- hash/md5/$i
    [m6] => 0.51757597923279                 <--- md5/$i
    [m7] => 0.90692067146301                 <--- hash/sha1/$i
    [m8] => 0.74792814254761                 <--- sha1/$i

)

<?php

$s="";
for ($i=0;$i<1000000;$i++)
$s.=$i;
$time=microtime(1);
   hash('md5', $s);
echo microtime(1)-$time,': hash/md5<br>';

$time=microtime(1);

 md5($s);
echo microtime(1)-$time,': md5<br>';

$time=microtime(1);
hash('sha1', $s);
echo microtime(1)-$time,': hash/sha1<br>';

$time=microtime(1);
sha1($s);
echo microtime(1)-$time,': sha1<br>';
?>

0.015523910522461: hash/md5
0.01521897315979: md5
0.020196914672852: hash/sha1
0.020323038101196: sha1