If you use (or are trying to) the very excellent Roundcube but you keep getting the white screen of death, then here's something to try.

As a quick test, disable the curl extension in your /etc/php.ini

;extension = curl.so

and restart your webserver…

sudo service httpd restart

If that seems to solve the problem (you can see the login screen and not a whitescreen) then try the fix that worked for me. Move curl.so after imap.so.

extension=imap.so
extension=curl.so

Your mileage may vary.