How to add a custom User Provider in Laravel 5.4

When you don't want to create a separate class file, customizations of EloquentUserProvider can be done by using an anonymous class inside the boot method of your service provider.

use Illuminate\Auth\EloquentUserProvider;
use Illuminate\Contracts\Auth\Authenticatable;

Auth::provider('eloquent', function($app, array $config) {
    return new class($app['hash'], $config['model']) extends EloquentUserProvider
    {
        // override any public methods here, e.g.
        public function validateCredentials(Authenticatible $user, array $credentials)
        {
            // some custom validation
        }
    };
});

None of the solutions I found quite worked for me, so I figured out a super easy way. (Laravel 5.4)

My problem was that I had to allow users to log in with their phone number too.

I override the EloquentUserProvider:

class PhoneUserProvider extends EloquentUserProvider
{
    public function retrieveByCredentials(array $credentials)
    {
        if (!$credentials) {
            return null;
        }

        // Fallback to original email authorization
        if (filter_var($credentials['email'], FILTER_VALIDATE_EMAIL)) {
            return parent::retrieveByCredentials($credentials);
        }

        // Assume we are logging in with a phone number
        return UserModel::where('phone', $credentials['email'])->first();
    }
}

And in my AppServiceProvider boot methode I added these lines:

Auth::setProvider(new PhoneUserProvider(app(Hasher::class), UserModel::class));

You can even set provider in login controller, based on some conditions, etc.

as @Mark Baaijens say

In your AuthServiceProvider

public function boot()
    {
        $this->registerPolicies();

        Auth::provider('eloquent', function($app, array $config) {
            return new class ($app['hash'], $config['model']) extends EloquentUserProvider {

                public function retrieveById($identifier)
                {
                    $model = $this->createModel();

                    $condition = [$model->getAuthIdentifierName() => $identifier];


                    if ($model instanceof User) {
                        $condition['custom_more_filed'] = 1;
                    }

                    return $model->newQuery()
                                 ->where($condition)
                                 ->first();
                }
            };
        });
    }

In case it is of any help to others looking for a way to customize the retrieveByCredentials method, it is possible to do so without implementing a custom UserProvider. The EloquentUserProvider simply takes a credentials array and builds out a User query using where clauses for each key/value pair in the array. So if you already have a class that is defining those credentials, you can simply customize there.

In my case, I had a ResetPasswordController that implemented ResetsPasswords. I was looking to modify how the user was selected during submission (using an id and password, rather than email and password), so I overrode the credentials method, replacing email with ID_Contact like so:

/**
 * Get the password reset credentials from the request.
 *
 * @param  \Illuminate\Http\Request  $request
 * @return array
 */
protected function credentials(Request $request)
{
    return $request->only(
        'password',
        'password_confirmation',
        'token',
        'ID_Contact'
    );
}

There were additional places that needed to be customized within the ResetsPasswords implementation in order to bring my particular use case into practice (plus the password reset form itself). Each scenario would be a little different when taking this approach. But this may point some in a helpful direction, depending on your particular requirements.