How can I generate unique alphanumeric?

function random_num($size) {

$alpha_key = ”;

$keys = range(‘A’, ‘Z’);

for ($i = 0; $i < 2; $i++) {

$alpha_key .= $keys[array_rand($keys)];

}

$length = $size – 2;

$key = ”;

$keys = range(0, 9);

for ($i = 0; $i < $length; $i++) {

$key .= $keys[array_rand($keys)]; }

return $alpha_key . $key; } echo random_num(9);