Generar una clave aleatoria

<?php

function createRandomPassword() {

    $chars = "abcdefghijkmnopqrstuvwxyz023456789";
    srand((double)microtime()*1000000);
    $i = 0;
    $pass = '' ;

    while ($i <= 7) {
        $num = rand() % 33;
        $tmp = substr($chars, $num, 1);
        $pass = $pass . $tmp;
        $i++;
    }

    return $pass;

}

$password = createRandomPassword();
echo "La nueva clave es : $password";

?>