First, I would lower the initial memory to something like 400 or 500, you don’t need to put that much there.
Where did the log you provided come from? It doesn’t really show anything that can explain the situation. Can you explain exactly how it crashes? Does it shut down after you start it or does it do something else?
Can you go into the Qubes Manager, right click on the qube and click on “Logs”? You will get all logs related to the qube and the ability to copy them. Provide the “guest-dev_PHP.log” one which contains logs that are related to the boot process and all services.