Initialize well known type values (#6713)

* Initialize well known type values

Also fixes #6667

* Revert unwanted change

php/tests/encode_decode_test.php

diff --git a/php/tests/encode_decode_test.php b/php/tests/encode_decode_test.php
index f2a5018..26b7656 100644
--- a/php/tests/encode_decode_test.php
+++ b/php/tests/encode_decode_test.php
@@ -5,6 +5,7 @@
 
 use Google\Protobuf\RepeatedField;
 use Google\Protobuf\GPBType;
+use Foo\TestStringValue;
 use Foo\TestAny;
 use Foo\TestEnum;
 use Foo\TestMessage;
@@ -167,6 +168,14 @@
         $this->assertSame("\"a\"", $m->serializeToJsonString());
     }
 
+    public function testEncodeStringValue()
+    {
+        $m = new TestStringValue(['field' => new StringValue(['value' => ''])]);
+        var_dump($m->getField());
+        var_dump($m->serializeToJsonString());
+        $this->assertSame("{\"field\":\"\"}", $m->serializeToJsonString());
+    }
+
     public function testDecodeTopLevelBytesValue()
     {
         $m = new BytesValue();