[10.x] Handle missing translation strings using callback

src/Illuminate/Translation/Translator.php

@@ -65,6 +65,20 @@ class Translator extends NamespacedItemResolver implements TranslatorContract
      */
     protected $stringableHandlers = [];
 
+    /**
+     * The callback that is responsible for handling missing translation keys.
+     *
+     * @var callable|null
+     */
+    protected $missingTranslationKeyCallback;
+
+    /**
+     * Indicates whether missing translation keys should be handled.
+     *
+     * @var bool
+     */
+    protected $handleMissingTranslationKeys = true;
+
     /**
      * Create a new translator instance.
      *
@@ -153,6 +167,10 @@ public function get($key, array $replace = [], $locale = null, $fallback = true)
                     return $line;
                 }
             }
+
+            $key = $this->handleMissingTranslationKey(
+                $key, $replace, $locale, $fallback
+            );
         }
 
         // If the line doesn't exist, we will return back the key which was requested as
@@ -308,6 +326,48 @@ protected function isLoaded($namespace, $group, $locale)
         return isset($this->loaded[$namespace][$group][$locale]);
     }
 
+    /**
+     * Handle a missing translation key.
+     *
+     * @param  string  $key
+     * @param  array  $replace
+     * @param  string|null  $locale
+     * @param  bool  $fallback
+     * @return string
+     */
+    protected function handleMissingTranslationKey($key, $replace, $locale, $fallback)
+    {
+        if (! $this->handleMissingTranslationKeys ||
+            ! isset($this->missingTranslationKeyCallback)) {
+            return $key;
+        }
+
+        // Prevent infinite loops...
+        $this->handleMissingTranslationKeys = false;
+
+        $key = call_user_func(
+            $this->missingTranslationKeyCallback,
+            $key, $replace, $locale, $fallback
+        );
+
+        $this->handleMissingTranslationKeys = true;
+
+        return $key;
+    }
+
+    /**
+     * Register a callback that is responsible for handling missing translation keys.
+     *
+     * @param  callable|null  $callback
+     * @return static
+     */
+    public function handleMissingKeysUsing(?callable $callback)
+    {
+        $this->missingTranslationKeyCallback = $callback;
+
+        return $this;
+    }
+
     /**
      * Add a new namespace to the loader.
      *

tests/Integration/Translation/TranslatorTest.php

@@ -40,4 +40,19 @@ public function testItCanCheckLanguageExistsHasFromLocaleForJson()
         $this->assertFalse($this->app['translator']->hasForLocale('1 Day'));
         $this->assertTrue($this->app['translator']->hasForLocale('30 Days'));
     }
+
+    public function testItCanHandleMissingKeysUsingCallback()
+    {
+        $this->app['translator']->handleMissingKeysUsing(function ($key) {
+            $_SERVER['__missing_translation_key'] = $key;
+            return 'callback key';
+        });
+
+        $key = $this->app['translator']->get('some missing key');
+
+        $this->assertSame('callback key', $key);
+        $this->assertSame('some missing key', $_SERVER['__missing_translation_key']);
+
+        $this->app['translator']->handleMissingKeysUsing(null);
+    }
 }