We use open_basedir as one of the measures to ensure websites are contained within their directories and prevent other users from being able to access your files.
However, as far as I can tell, this is causing some issues with PHAR files, so it appears that they don’t work with our current setup.
We should investigate this, but you may be able to work around it in the mean time.
Could you perhaps try to extract the .PHAR file on your computer and just put the extracted files in a directory? If you rename the .phar to .zip, you should just be able to extract it. And as far as I can tell, osTicket also supports loading plugins from directories.