There's no harm in knowing more than one language. In fact it's a good thing as it will introduce you to new approaches and techniques, and open your eyes to the limitations or design flaws in your current language of choice (or which there are always some).

I've always been suspicious of PHP as a language due to its carbuncle nature and poor design (IMO) and while I appreciate the flexibility of it and that you can use one of the many frameworks or OS applications built with it to great success, ultimately when you have to drop down into the language you'll feel the pain.

PHP 5 lessens this to some degree I suppose.

With your OO background none of the popular languages are going to present much of a problem so why not take a look at both Ruby and Python and decide which you prefer to work with? If you like neither then stick with PHP.

The frameworks are not as important as the underlying languages IMO.