Can I work as a software engineer and be a PhD student?
I did combine PhD and work for 4 years, so, theoretically - yes, it's possible. But I didn't like the process because I wasn't fully invested in either activity. It also significantly delayed my defense. It depends on how productive you are. At times you will be facing certain deadlines and will be feeling torn between two major activities. 

I think you should go ahead with PhD if you are serious about it. It will take some 3-6 years of your time, so why to postpone it? I think it's totally possible to get internships at summer time and you may have to develop software as a part of your research, so you'll be practicing your skills. 

You can also consider free-lance work so that you have more control over your time.