$db = new Database(new MySQLi('localhost', 'root', '', 'test'));


$user = $db->query('SELECT * FROM users WHERE username = :username AND id = :id', [
    'username' => 'Test',
    'id' => 1
], function() use ($db) {
    return $db->query('SELECT * FROM users WHERE username = :username', ['username' => 'Test'])->num_rows == 0;
});

echo $multiQuery = $db->multiQuery([
    [
        'query' => 'SELECT * FROM users WHERE username = :username AND id = :id',
        'params' => [
            'username' => 'iExit',
            'id' => 1
        ]
    ],
    [
        'query' => 'INSERT INTO users SET username = :username',
        'params' => [
            'username' => 'Test1231'
        ],

        'callback' => function() use ($db) {
        //callback :D
            return $db->query('SELECT * FROM users WHERE username = :username', ['username' => 'Test1231'])->num_rows == 0;
        }
    ]

])[0]->fetch_object()->username; // gibt "iExit" aus, war nur ein Test und ich würde sowas niemals nutzen.