[error-handling] Where does PHP store the error log? (php5, apache, fastcgi, cpanel)

The best way is to look in your httpd.conf file and see what the default is. It could also be overridden by your specific virtual host. I start by looking at /etc/httpd/conf/httpd.conf or /etc/apache2/httpd.conf and search for error_log. It could be listed as either /var/log/httpd/error_log or /var/log/apache2/error_log but it might also be listed as simply logs/error_log.

In this case it is a relative path, which means it will be under /etc/httpd/logs/error_log. If you still can't find it check the bottom of your httpd.conf file and see where your virtual hosts are included. It might be in /etc/httpd/conf.d/<- as "other" or "extra". Your virtual host could override it then with ErrorLog "/path/to/error_log".