Applying for a PhD - how and when to broach the fact that I have previously left a PhD program
For a variety of reasons you should reveal it early as part of your education in the CV or other materials. But you don't need to give any reason there. Just that you were in a doctoral program (and maybe ABD) from such a date to some later date. 

You are probably required to give transcripts of all educational endeavors, and it would be a lapse to omit any. 

There are a lot of reasons for leaving a doctoral program and only of a few of them reflect badly on the student. Not passing comprehensives might be a negative and being expelled for ethical lapses would certainly be. But those are only a few of the possibilities, including just becoming disillusioned with the program or with an advisor. Money. Family. Medical. 

But later, in an interview, you will probably be asked and then is the time, IMO, to give reasons. 

And, if you think the experience strengthens your case, then, in a Statement of Purpose you could say how it supports your future goals.