Php 7.3 fix (#5434)

* Inherit from message instead of implement

When regestering class (implmenets other class) during MINIT, zend_class_implements
would call zend_class_entry->interface_gets_implemented(). In PHP-7.3 interface_gets_implemented shares the same location with create_object.
However, during MINIT, the global object storeage hasn't been initialized. And thus, caused segment fault in php 7.3.

* Use zend_string_init where interned string may be the value.
zend_string_dup will keep using the existing interned string.
In php 7.3, interned string cannot be destroyed from user's code.

* Uncommment debug code

* Use latest phpunit for each php versions

* Revert change in Dockerfile

* Update php test to use the new docker image

* Update composer

* Change docker organization

* Update phpunit

* Debug phpunit

* Store phpunit into bin dir in docker image

* Install valgrind to docker

* Fix compatibility test

* Remove generated_service_test from compatibility c extension test

* Update 32bit php test to the new docker image

* Install bison

* Fix build.sh

* Fix DOCKERIMAGE_PREFIX

* Fix basename

* Add comment to build_and_run_docker2.sh

* Remove commented code

* Fix comments

php/tests/encode_decode_test.php

diff --git a/php/tests/encode_decode_test.php b/php/tests/encode_decode_test.php
index 8168eeb..06f9a90 100644
--- a/php/tests/encode_decode_test.php
+++ b/php/tests/encode_decode_test.php
@@ -31,6 +31,7 @@
     {
         $m = new TestMessage();
         $m->mergeFromJsonString("{\"optionalInt32\":1}");
+        $this->assertEquals(1, $m->getOptionalInt32());
     }
 
     public function testDecodeTopLevelBoolValue()
@@ -276,6 +277,7 @@
         $to = new TestPackedMessage();
         $to->mergeFromString(TestUtil::getGoldenTestPackedMessage());
         TestUtil::assertTestPackedMessage($to);
+        $this->assertTrue(true);
     }
 
     public function testPackedDecodeUnpacked()
@@ -283,6 +285,7 @@
         $to = new TestPackedMessage();
         $to->mergeFromString(TestUtil::getGoldenTestUnpackedMessage());
         TestUtil::assertTestPackedMessage($to);
+        $this->assertTrue(true);
     }
 
     public function testUnpackedEncode()
@@ -298,6 +301,7 @@
         $to = new TestUnpackedMessage();
         $to->mergeFromString(TestUtil::getGoldenTestPackedMessage());
         TestUtil::assertTestPackedMessage($to);
+        $this->assertTrue(true);
     }
 
     public function testUnpackedDecodeUnpacked()
@@ -305,6 +309,7 @@
         $to = new TestUnpackedMessage();
         $to->mergeFromString(TestUtil::getGoldenTestUnpackedMessage());
         TestUtil::assertTestPackedMessage($to);
+        $this->assertTrue(true);
     }
 
     public function testDecodeInt64()
@@ -361,6 +366,7 @@
         $data = hex2bin('c80501');
         $m = new TestMessage();
         $m->mergeFromString($data);
+        $this->assertTrue(true);
     }
 
     public function testEncodeNegativeInt32()