WARNING: "pecl/mongo" is deprecated in favor of "channel:///mongodb"
What does that pecl message mean? I mean specifically how does one use the channel described in the error message? ... or in other words how can I address this error (but not specific to mongo, i.e. I get this failure not just when trying to install Mongo but also when trying to install other things and I don't understand how to fix it in general).
Should I be "installing" the channel via pecl somehow? (Much like one adds a channel to yum).
It simply means that mongo extension is deprecated, and you should use mongodb instead:
pecl install mongodb