php/smoketest.php: Iterating foreach ($it as $k => $c) doesn't work
unless $k is a string or integer in PHP < 5.5, so test the old way
under older versions.

git-svn-id: file:///home/xapian-svn/xapian/trunk@18325 6b2f1b92-a4f3-0310-943f-de3cbe41748b