Ignore unknown enum value when ignore_unknown specified (#7455)

php/tests/encode_decode_test.php

diff --git a/php/tests/encode_decode_test.php b/php/tests/encode_decode_test.php
index 319e332..5442f50 100644
--- a/php/tests/encode_decode_test.php
+++ b/php/tests/encode_decode_test.php
@@ -884,6 +884,11 @@
         $m->mergeFromJsonString("{\"unknown\":{\"a\":1, \"b\":1},
                                 \"optionalInt32\":1}", true);
         $this->assertSame(1, $m->getOptionalInt32());
+
+        // Test unknown enum value
+        $m = new TestMessage();
+        $m->mergeFromJsonString("{\"optionalEnum\":\"UNKNOWN\"}", true);
+        $this->assertSame(0, $m->getOptionalEnum());
     }
 
     public function testJsonEncode()