Sorry for not examining the source earlier.
1) I do not think you should force the use of PEAR_Config. It should be upto the user to make use of PEAR_Config if they so choose. Then again it might be the best way to get some kind of default and it seems to be user changeable.
2) it would be better if you would extend your own error class and define error codes so that the error conditions can be controled programmatical in more detail.
3) some minor CS issues