It\'s been working for ages and stopped. I must be missing something obvious so figured stack overflow might be able to help.
/etc/php5/apache2/php.ini relevant set
It ended up being permission issues. The file grew too big to open quickly, so I deleted and recreated it, but with read only permissions.
Totally forgot I deleted it, what a pain.
Did you restart the Web server? Until you do php.ini changes are not considered.
Also, if you want to track PHP errors, you need to have track_errors=On and error_reporting=E_ALL, although that is not related with error_log calls. Also make sure that the error log file is writeable by the Web server user.
In my php.ini file (Xampp default), there was a second entry for error_log overriding the one that I had set.
So if the other answers do not work for you too, search again for multiple entries of the necessary settings.