Stick to PHP if you already know it. It's not worth it to learn a different one unless you have serious reasons.
You can use PHP to build desktop apps - check PHP GTK2 project. If you plan to sell it, you should know it is probably easier to crack. But then again, anything is crackable.
You will have to develop your own scraping framework as there is no solid one for PHP.
If you do want to learn a language, I'd recommend Python. It is possibly the most elegant language I have ever seen and can be used from Web apps to scientific apps. Also, Python has "mechanize" class which is a port of Perl's www-mechanize which basically is a programmable "web browser" that can scrape and submit forms, follow links, etc.
Also, if you want to build bots, get familiar with regex and DOM parsers. Personally in PHP i use phpQuery for DOM manipulation as it is a PHP port of JQuery and is easy to use.