@gnusocialjp
I didn't have any issue with it conflicting with separately installed PEAR packages, it was more to do with issues internal to MDB2 itself.
It was this issue that I was running into: https://notabug.org/diogo/gnu-social/issues/248
Meanwhile it was possible to getting around it using this very hackish workaround: https://notabug.org/diogo/gnu-social/pulls/252/files (I'm assuming something returned false in error, and that's just sidestepping it).
Either way, it may be appropriate to just gut out MDB2 and work with PHP PDO directly as part of some DB code refactoring, but I understand that could be a monumental task. If there's interest in such an effort, I could try jumping into working on something like that perhaps.
I think the inclusion of MDB2 was probably for an unfinished idea back in StatusNet days to have an ORM-like interface with the database, but never really finished, but that's just my assumptions in a cursory review of the code when I was trying to pin down the installer issue.