Can I extend my e visa from Phetchabun, or do I have to leave Thailand for an extension?

Where do you come from?

What type of visa do you have? Every visa is an evisa.

You can not extend a 90 day visa for 30 days if you have a 90 day visa you may have a tourist visa which is valid for 90 days which you would be stamped in for 60 days this can be extended by 30 days

if you got a 90-days Non-Imm-O visa type, you cannot extend it for one month. You need to exit Thailand and re-enter visa-exempt, which will get you stamped in for 60 days. Entry allowance or denial however is connected to your overall entry history in the recent 365 days - if you have too many visa-exempt entry stamps in your passport, an Immigration official might think you are misusing touristic entries for a longstay. So it's advised to be prepared to show the three proofs: accomodation for a few nights, 20,000 THB or equivalent in cash, and an onward travel proof out of Thailand within the 60 days you will get stamped in