Knowing PHP is extremely useful. Most good open source scripts I use are written in PHP, so it's nice being able to make changes without much hassle.
I've played with and liked Perl and Ruby as well.
Great book:
Webbots, Spiders, And Screen Scrapers: A Guide to Developing Internet Agents With Php/ Curl - Michael Schrenk - Paperback (ISBN 9781593271206) - Buy Books, Music and Movies at Borders
Very simple free PHP tutorials:
PHP Tutorial - Introduction
I've played with and liked Perl and Ruby as well.
Great book:
Webbots, Spiders, And Screen Scrapers: A Guide to Developing Internet Agents With Php/ Curl - Michael Schrenk - Paperback (ISBN 9781593271206) - Buy Books, Music and Movies at Borders
Very simple free PHP tutorials:
PHP Tutorial - Introduction