[php] Fix widespread use of bare strings in testsuite

These generate warnings with PHP 7.3, which will become errors in a
future version.

Examples/test-suite/php/abstract_inherit_ok_runme.php

@@ -3,7 +3,7 @@
 require "tests.php";
 require "abstract_inherit_ok.php";
 
-check::classes(array(Foo,Spam));
+check::classes(array('Foo','Spam'));
 $spam=new Spam();
 
 check::equal(0,$spam->blah(),"spam object method");