session_start();

echo $_SESSION['temparray'][] = generateCode();

var_dump($_SESSION['temparray']);

function generateCode(){
    $code = strtoupper(substr(md5(crypt(uniqid(microtime()))), -5));
    if(is_numeric($code)||in_array($code, $_SESSION['temparray'])){
        return generateCode();
    }
    return $code;
}