I followed the instructions on upgrading but am running into the "Fatal error: Call to undefined function mb_http_output() " error I see a few people have reported. I run the site on a cPanel account which I have full WHM root access to check the PHP config. The problem with the php extension mbstring not being enabled does not seem to be the problem for me. Does anyone have experience of WHM/cPanel upgrades? Or any recommendations on an alternative way to upgrade an existing site?

I've checked the two new directories framework and cms have correctly set up permissions. All 0755

If you make a PHP script and add phpinfo(); into it, does mbstring show up as a loaded extension in the output?

Thanks. Did that and although Apache showed the mbstring, php.ini did not. I rebuilt apache making sure it was enabled (using EasyApache in WHM/cPanel just in case that's useful for someone) and it's working fine now.