如何摆脱PHP中的功能

I have the following function:

function authenticate($req, $resp, $args) {
    $credentials = json_decode($req->getBody());

    $sql = "SELECT usr_password FROM ict_users WHERE usr_username='".$credentials->username."'";
    try {
        $db = DB_Connection();
        $stmt = $db->query($sql);  
        $password = $stmt->fetchAll(PDO::FETCH_OBJ);
        $db = null;
    } catch(PDOException $e) {
        echo '{"error":{"text":'. $e->getMessage() .'}}'; 
    }

    if (password_verify($credentials->password, $password[0]->usr_password)) {
        echo 'Valid password !';
    } else {
        echo 'Invalide password !.';
    }
}

If I put return; after $db = null; will it stop executing my function? Or is there a better way to stop the function when an error is caught?

You have an try {}catch(){} block, why not throw new Exeption(); instead of return null. And return null is not done if you want "stop the function when an error is caught", you just returning null from the function.