Assuming you're using internal web pages (all in one page), there's examples in the documentation about how to do that. I don't know how good that documentation is, though, because I don't use that model myself. If you're using the multiple page model, it's somewhat more complicated, and isn't really documented. The gist of the latter approach, though, is to bind an appropriate page event and use a script to extract the parameters manually from the URL. There are other approaches that people have used, like putting stuff in local storage. If it works for your app, I think you can also turn off data-Ajax on everything, but then you lose a lot of the value of jQM in my opinion.
Bottom line, though, is that jQM doesn't really support this in a reliable, well-documented manner, so you'll have to read up on it a lot to fully understand it, if that's even possible given how it works. I may be wrong on that for internal pages, but like i said I don't really use those; dealing with all the hash stuff just got too confusing. My recommendation, after spending the better part of the past two days doing that reading myself, is to just pick an approach early on knowing that there is no "right" answer out there that you'll find if you dig far enough. If you're curious, and using external pages, I posted another thread asking a question about which event to bind in this model that has the snippet for parsing the parameter from the URL, one of many that I had found. If you read that, I think you can get a good general idea of the approach.
I don't do PHP much, but it looks like that error is due to a missing variable. Since "missing parameters" is a possible problem in any web app, you probably want to put something in to deal with that error state (I know you probably haven't gotten that far yet).
Hope this helps.