Check if variables contain identical value

I have problem to check either any of my variable contains identical value. Assume I have 8 variable with random generated lowercase alphabet only. For your info, this value don't need to be secure.

$variable1 = random_string();
$variable2 = random_string();
$variable3 = random_string();
$variable4 = random_string();
$variable5 = random_string();
$variable6 = random_string();
$variable7 = random_string();
$variable8 = random_string();

function random_string($length = 6) {
    $characters = 'abcdefghijklmnopqrstuvwxyz';
    $characters_length = strlen($characters);
    $random_string = '';
    for ($i = 0; $i < $length; $i++) {
        $random_string .= $characters[rand(0, $characters_length - 1)];
    }
    return $random_string;
}

I have seen this accepted answer at this question. My question, is there any simpler method to compare all this 8 variables if any of this variables have same or identical value? And if it do have same value, then regenerate another random string until none of it contain same value. Thanks.