I have a 60 day single entry Visa. I've been here about ten days. If I travel to Siem Reap, will it become invalid and I only get a thirty day stamp on returning?
TLDR : Answer Summary
When you travel to Siem Reap with your 60-day single entry visa, it will indeed become invalid once you exit Thailand. Upon re-entering, you will receive a 30-day visa exemption. If you plan to stay longer than that, you can extend your stay by an additional 30 days at an immigration office for a fee.