[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/import_nomodule_runme.php

@@ -3,9 +3,9 @@ require "tests.php";
 require "import_nomodule.php";
 
 // No new functions
-check::functions(array(create_foo,delete_foo,test1,is_python_builtin));
+check::functions(array('create_foo','delete_foo','test1','is_python_builtin'));
 // No new classes
-check::classes(array(import_nomodule,Bar));
+check::classes(array('import_nomodule','Bar'));
 // now new vars
 check::globals(array());