// Get salary for user id: 10
$salary = Salary::getSalary([10], '2018-12-01', '2018-12-31');

echo $salary->user->fullname;
echo $salary->salary->total;
echo $salary->checked;

// Get salary for users - 10, 20, 30
$salaries = Salary::getSalary([10, 20, 30, '2018-12-01', '2018-12-31');

foreach ($salaries as $salary) {
    echo $salary->user->fullname;
    echo $salary->salary->total;
    echo $salary->checked;
}

public function getSalary($user_id, $date_from, $date_to)
{
    $salary = new stdClass;

    if ( count($user_id) == 1 ) {
        $salary->user = new stdClass;
        $salary->user->fullname = self::getUserById($user_id)->fullname;
        $salary->user->phone    = self::getPhone($user_id);
        $salary->user->email    = self::getUserById($user_id)->e_mail;

        $salary->salary      = self::getSalary($user_id, $date_from, $date_to);
        $salary->checked     = self::isChecked($user_id, $date_from, $date_to);
    } else if ( count($user_id) > 1 ) {
        foreach ($user_id as $employee)
        {
            $salary->employee = new stdClass;
            $salary->employee->fullname = self::getUserById($user_id)->fullname;
            $salary->employee->phone    = self::getPhone($user_id);
            $salary->employee->email    = self::getUserById($user_id)->e_mail;
            $salary->employee->siawork  = self::getUserById($user_id)->siawork;

            $salary->salary      = self::getSalary($user_id, $date_from, $date_to);
            $salary->checked     = self::isChecked($user_id, $date_from, $date_to);
        }
    }

    return $salary;
}

public function getSalary($user_id, $date_from, $date_to)
{
    //Return an array of salary objects to match your array of id inputs
    $salary = [];

    if ( ! is_array($user_id) ) {
        //In case a non-array is entered
        $user_id = [$user_id];
    }

    // A foreach will work even if there's only one user ID given,
    // as long as it's an array (which it should be by this point)
    foreach ($user_id as $id)
    {
        //Get your salary object by id
        $this_user = new stdClass;
        $this_user->user = new stdClass;
        $this_user->user->fullname = self::getUserById($id)->fullname;
        $this_user->user->phone    = self::getPhone($id);
        $this_user->user->email    = self::getUserById($id)->e_mail;

        $this_user->salary      = self::getSalary($id, $date_from, $date_to);
        $this_user->checked     = self::isChecked($id, $date_from, $date_to);

        //Put it in the output array
        $salary[] = $this_user;
    }

    return $salary;
}

public function getSalary($user_id, $date_from, $date_to)
{
    $salaries = [];

    if ( !is_array($user_id) ) {
        $user_id = [$user_id];
    }
    foreach ($user_id as $employee)
    {
        $salary->employee = new stdClass;
        $salary->employee->fullname = self::getUserById($user_id)->fullname;
        $salary->employee->phone    = self::getPhone($user_id);
        $salary->employee->email    = self::getUserById($user_id)->e_mail;
        $salary->employee->siawork  = self::getUserById($user_id)->siawork;

        $salary->salary      = self::getSalary($user_id, $date_from, $date_to);
        $salary->checked     = self::isChecked($user_id, $date_from, $date_to);

        $salaries[] = $salary;
    }

    return $salaries;
}

if ( count($user_id) == 1 ) {
        $salaries = $salaries[0];
    }
    return $salaries;
}