<?php

function getRandomString($length = 8) {
    $characters = str_split('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789');    

    for ($i = 0; $i < $length; $i++) {
        $rand = '';
        foreach (array_rand($characters, 5) as $k) $rand .= $characters[$k];
        $string[$i] = $rand;
    }

    return $string;
}

print_r(getRandomString());