So... there is an injunction that is true in all cases of job-hunting, but that is particularly true in cases like yours, where the information is mixed. Your resume must tell a story. When the recruiter reads your resume, they're going to be trying to put together in their minds an idea of who you are as a person and, more importantly, who you're likely to be as an employee. You need to get out ahead of that game by looking at the facts on the ground, figuring out a not-untrue story to tell about yourself, and building a resume that tells it. Worth remembering that this isn't just the story that gets you in the door. It's the story you'll want to live while you're there. Tweaking this story is also a good way to get companies that won't work well for you to not take you, and save frustration on both sides. If you can only get a job by convincing the employer that you are an absolute rockstar, and you can't back that up without breaking yourself on the stress, you might not want that job.
So, let's look at stories you could tell.
- "I tried to go for a PhD, but I couldn't hack it and gave up". That's a seriously negative way to approach it, and it makes you look worse than you are. It specifically throws you in the same pot as the guys who were going for a PhD full-time and gave up, without reference to the fact that you were going full-time at the time. That's not giving yourself enough credit.
- Leave it off entirely: no effect one way or the other. You're not hurting yourself, but I suspect we can do better.
- "I'm a solid, full-time worker. Once, when I was younger and more ambitious, I tried to manage full-time work and a PhD at the same time. Didn't make it. I've mellowed a bit since then." Sets you up as the wiser, calmer sort. You know a lot, and you can put in the work, but they shouldn't expect you to be on-fire levels of dynamic and driven.
- "I'm a solid, full-time worker, and I've put in a fair amount of effort on my own time in professional development. For example, between year X and Y, I took a whole bunch of applicable graduate courses in the evenings." This one ignores the idea that there was a "PhD program" or particular expectation of continuation at all. It positions you as someone who's going to keep going ongoing professional development in your personal time, if perhaps in other ways.
Of course, the resume is on the first step on this one. The interview keeps it going If you mention the classes at all, they'll likely ask you why you started, and why you stopped. You'll want an answer for that that both is true and supports the narrative that your resume is building. That's the trick, though - there are many things that one could say that would be true, and there's no way to say them all. It's worthwhile, then, to pick and choose... and don't let your story buy you a position that you won't be able to handle.