Function that allows the user to log in with both the username and the email.

LoginController.php

<?php

class LoginController extends Controller
{

    (...)

    /**
    * Checks if the parameter passed at login is a username or an email account.
    * Depending on the check it will return one login method or another
    *
    * @override
    * @return string
    */
    public function username()
    {
        $loginType = request()->input('username');
        $this->username = filter_var($loginType, FILTER_VALIDATE_EMAIL) ? 'email' : 'username';
        request()->merge([$this->username => $loginType]);

        return property_exists($this, 'username') ? $this->username : 'email';
    }
}