Cleaning "infected" file from the odmarco string

I use DreamHost for quite some sites, both my own and of friends. Recently I got informed that one of my sites was triggering a warning in anti-virus software and when I went to investigate, it turned out quite a few sites got hit by an annoying script that inserts a hidden iframe. Removing all those by hand would be an annoying job, so I decided to automate it. Given that I don't know much shell scripting but have a firm knowledge of PHP, I decided to give that a go.