Add check for binary data in FCM data fields

CHANGELOG.md

@@ -1,6 +1,10 @@
 # CHANGELOG
 
 ## [Unreleased]
+### Changed
+* Message data added to a with `CloudMessage::withData()` now rejects binary data to avoid broken
+  messages being sent to the Firebase API.
+  ([#441](https://github.com/kreait/firebase-php/issues/441))
 
 ## [5.5.0] - 2020-06-19
 ### Added

src/Firebase/Messaging/MessageData.php

@@ -27,6 +27,13 @@ public static function fromArray(array $data): self
                 throw new InvalidArgumentException('Message data must be a one-dimensional array of string(able) keys and values.');
             }
 
+            if (self::isBinary((string) $value)) {
+                throw new InvalidArgumentException(
+                    "The message data field '{$key}' seems to contain binary data. As this can lead to broken messages, "
+                    .'please convert it to a string representation first, e.g. with bin2hex() or base64encode().'
+                );
+            }
+
             $messageData->data[(string) $key] = (string) $value;
         }
 
@@ -48,4 +55,9 @@ private static function isStringable($value): bool
     {
         return \is_null($value) || \is_scalar($value) || (\is_object($value) && \method_exists($value, '__toString'));
     }
+
+    private static function isBinary(string $value): bool
+    {
+        return \mb_detect_encoding($value) === false;
+    }
 }

tests/Unit/Messaging/MessageDataTest.php

@@ -59,6 +59,9 @@ public function __toString()
                     }
                 }],
             ],
+            'UTF-8 string' => [
+                ['key' => 'Jérôme'],
+            ],
         ];
     }
 
@@ -68,6 +71,10 @@ public function invalidData()
             'nested array' => [
                 ['key' => ['sub_key' => 'sub_value']],
             ],
+            // @see https://github.com/kreait/firebase-php/issues/441
+            'binary data' => [
+                ['key' => hex2bin('81612bcffb')], // generated with \openssl_random_pseudo_bytes(5)
+            ],
         ];
     }
 }