<?php
$vectorA = range(0,1024);
$vectorB = range(0,1024);

$start = microtime(true);
$sum = 0;
for ($i = 0; $i < 1024; $i++) {
    $sum += pow(($vectorA[$i] - $vectorB[$i]), 2);
}
$sum = sqrt($sum);

$end = microtime(true)-$start;

echo $end." seconds"; //0.0011429786682129 seconds