Format the callback URI, resolving a relative URI if needed

composer.json

@@ -11,7 +11,8 @@
     }],
     "require": {
         "php": "^5.6 || ^7.0",
-        "laravel/socialite": "~3.0|~4.0"
+        "laravel/socialite": "~3.0|~4.0",
+        "illuminate/support": "~5.4|~5.7.0|~5.8.0"
     },
     "require-dev": {
         "phpunit/phpunit": "^5.0",

src/Config.php

@@ -2,6 +2,9 @@
 
 namespace SocialiteProviders\Manager;
 
+use Illuminate\Support\Str;
+use Illuminate\Support\Facades\URL;
+
 class Config implements Contracts\ConfigInterface
 {
     /**
@@ -22,10 +25,23 @@ public function __construct($key, $secret, $callbackUri, array $additionalProvid
         $this->config = array_merge([
             'client_id'     => $key,
             'client_secret' => $secret,
-            'redirect'      => $callbackUri,
+            'redirect'      => $this->formatRedirectUri($callbackUri),
         ], $additionalProviderConfig);
     }
 
+    /**
+     * Format the callback URI, resolving a relative URI if needed.
+     *
+     * @param  string  $callbackUri
+     * @return string
+     */
+    protected function formatRedirectUri(string $callbackUri)
+    {
+        return Str::startsWith($callbackUri, '/')
+                    ? URL::to($callbackUri)
+                    : $callbackUri;
+    }
+
     /**
      * @return array
      */