How do I handle academic references for US PhD program, when I have been out of academia for a (very) long time?
In contacting your old professors you might start by reminding them of what courses you took with them and what grades you earned in those courses. It would also be appropriate to supply a copy of your full academic transcript.  

I frequently get requests like this from students that I haven't seen in 10 years or more.  I comment on the students transcript and grades in my courses and whatever I can remember about the student.  This isn't ideal, but it's better than nothing.  

Another common strategy for students who've been working in industry for a long time is to begin graduate study by taking a few courses on a part-time basis at a local university.  It's much less of a commitment by the department to allow you to take a course or two than it would be to admit you as a full-time student and provide you with financial support.  If you do well in those courses then you can ask the instructors for academic references.