Trying to upgrade to 2.2.10.

"Check for writeable config file
The HTTP process cannot write to the config.php file. Please try to change the permissions on this file to 777 until the upgrade process is complete"

I have set chmod 777 config.php its -rwxrwxrwx but still the same error.

It may be an ownership issue rather than permission. You'd need to see what user PHP/phar is running as and compare it to the owner of the config.php file. It's possible you'll need to contact your host for assistance if you don't have shell access.

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Also in rare cases I've also seen that problem come up when there's other non-writable files in the site directory. I don't know if that's still a thing, but worth checking.

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Thanks for the reply. I have shell access, how do I check what user PHP/phar is running?

777 should be good.

Make sure the browser is refreshed, or start the upgrade from another (incognito) browser.