The PHP5 move also meant many hosts also upgraded to a fresh apache – 2.0.62? Now, many .htaccess rewrite rules failed to work and one of the main reasons that I could learn was that the previous rewrite engine was very forgiving with the regex rules and this new one hits right back at you!
RewriteCond %{HTTP_HOST} !^www.foodomain.com$ # was okay with the previous one but now you better have it as RewriteCond %{HTTP_HOST} !^www\.foodomain\.com$
Does that ring a bell?