Added a validator that checks, if a values is empty, if another one is set.

app/CustomValidator.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace App;
+
+use Illuminate\Validation\Validator;
+
+class CustomValidator extends Validator
+{
+    public function validateEmpty($attribute, $value)
+    {
+        return !$this->validateRequired($attribute, $value);
+    }
+
+    public function validateEmptyIf($attribute, $value, $parameters)
+    {
+        $key = $parameters[0];
+
+        if ($this->validateRequired($key, $this->getValue($key))) {
+            return $this->validateEmpty($attribute, $value);
+        }
+
+        return true;
+    }
+}

app/Providers/AppServiceProvider.php

@@ -3,6 +3,8 @@
 namespace App\Providers;
 
 use Illuminate\Support\ServiceProvider;
+use App\CustomValidator;
+use Validator;
 
 class AppServiceProvider extends ServiceProvider
 {
@@ -13,7 +15,9 @@ class AppServiceProvider extends ServiceProvider
      */
     public function boot()
     {
-        //
+        Validator::resolver(function ($translator, $data, $rules, $messages) {
+            return new CustomValidator($translator, $data, $rules, $messages);
+        });
     }
 
     /**