Comment 3 for bug 1579815

Just a quick update, based upon SRU precedence, I don't think this is likely to occur. Nothing in Trusty itself needs php5-mongodb, it doesn't exist in Debian (afaict) and the way the packaging works (rather intentionally afaict), it is explicitly only designed to work with PHP7+. Finally, there are two further dependencies, libbson and libmongoc which would also need backporting in order to achieve this and that adds to the SRU overhead and review(s).

You might consider setting up a PPA for your own use and seeing if you can get a php5-mongodb built there, but I don't think it will be straightforward.