Update PHP client default value (#417)

* update php default value

* update php samples with oas3

samples/client/petstore/php/OpenAPIClient-php/tests/EnumClassTest.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace OpenAPI\Client;
+
+use OpenAPI\Client\Model\EnumClass;
+
+class EnumClassTest extends \PHPUnit_Framework_TestCase
+{
+    public function testPossibleValues()
+    {
+        $this->assertSame(EnumClass::ABC, '_abc');
+        $this->assertSame(EnumClass::EFG, '-efg');
+        $this->assertSame(EnumClass::XYZ, '(xyz)');
+    }
+}