[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/arrayptr_runme.php

@@ -4,7 +4,7 @@ require "tests.php";
 require "arrayptr.php";
 
 // No new functions
-check::functions(array(foo));
+check::functions(array('foo'));
 // No new classes
 check::classes(array());
 // now new vars