[PHP] Improve: Make validation strict (#7724)

* Add test case which reproduce the problem

refs https://github.com/swagger-api/swagger-codegen/pull/7686#issuecomment-368200011
> 1. We should pass true as 3rd argument of in_array()

* Add test case for setter

* Strict validation

* Update samples

* Tweak expected value according to changes in #7723

samples/client/petstore/php/SwaggerClient-php/test/Model/EnumTestTest.php

@@ -83,6 +83,13 @@ class EnumTestTest extends \PHPUnit_Framework_TestCase
     {
     }
 
+    /**
+     * Test attribute "enum_string_required"
+     */
+    public function testPropertyEnumStringRequired()
+    {
+    }
+
     /**
      * Test attribute "enum_integer"
      */