diff --git a/php/src/Google/Protobuf/Internal/Message.php b/php/src/Google/Protobuf/Internal/Message.php
index 64aadf9d26..e74943c1ab 100644
--- a/php/src/Google/Protobuf/Internal/Message.php
+++ b/php/src/Google/Protobuf/Internal/Message.php
@@ -1188,6 +1188,7 @@ class Message
                 $v->mergeFromJsonArray($value, $ignore_unknown);
                 $fields[$key] = $v;
             }
+            return;
         }
         if (is_a($this, "Google\Protobuf\Value")) {
             if (is_bool($array)) {
@@ -1241,7 +1242,13 @@ class Message
             if (is_null($field)) {
                 $field = $this->desc->getFieldByName($key);
                 if (is_null($field)) {
-                    continue;
+                    if ($ignore_unknown) {
+                        continue;
+                    } else {
+                        throw new GPBDecodeException(
+                            $key . ' is unknown.'
+                        );
+                    }
                 }
             }
             if ($field->isMap()) {
diff --git a/php/tests/EncodeDecodeTest.php b/php/tests/EncodeDecodeTest.php
index d471a5a17a..b7c1ab24ca 100644
--- a/php/tests/EncodeDecodeTest.php
+++ b/php/tests/EncodeDecodeTest.php
@@ -45,6 +45,20 @@ class EncodeDecodeTest extends TestBase
         $this->assertEquals(1, $m->getOptionalInt32());
     }
 
+    public function testDecodeJsonUnknown()
+    {
+        $this->expectException(Exception::class);
+
+        $m = new TestMessage();
+        $m->mergeFromJsonString("{\"unknown\":1}");
+    }
+
+    public function testDecodeJsonIgnoreUnknown()
+    {
+        $m = new TestMessage();
+        $m->mergeFromJsonString("{\"unknown\":1}", true);
+    }
+
     public function testDecodeTopLevelBoolValue()
     {
         $m = new BoolValue();