I think that this would be great to have in PEAR. I use mpd myself and love it.
I agree with Christian's comments.
dirname(__FILE__) should never be used for including files. Include files properly via the include_path. (is_file isn't necessarily needed.) I suggest another class_exists after including the file.
Static properties shouldn't be used for connection properties. This precludes connecting to 2 MPD instances in the same script at the same time.
A PEAR error handling scheme should be used for errors. Since this is PHP5 code PEAR_Exception should be used. If you really want to collect errors instead use PEAR_ErrorStack.
The indentation seems very odd, please check and conform to the PEAR CS requirements.