How to log php error in a separate file?

I think that here it's just the generic Apache errors that are seen in /var/log/httpd/error_log.

I would insert a call to error_log('test'); into some PHP logic that is known to get interpreted when some specific page is refreshed, and if that's not seen in /var/log/php-errors.log then I'd suspect that the versions upgraded to have some kind of a different config or permissions scheme that made the old config stop working.

I'd recommend going through the 'Error handling and logging' section in /etc/php/php.ini, and checking for proper values for other directives. The directive error_reporting should be set to E_ALL or E_ALL & ~E_DEPRECATED & ~E_STRICT. The directive log_errors should be On.

If just a filename (but no path) is specified for error_log, then the file will appear in the same directory as the PHP script that calls error_log(...).

I'd also recommend specifying /tmp/php_error_log for error_log, refreshing the page, and then ls /tmp to see if the file is there.

If the file is not there, login as root, and do a search for the file name across the whole file system so see where it ended up.

Generally, the proper config to get PHP error log working in a particular directory changes across different versions of Apache, PHP, and various distros.

I can say that on Arch Linux with Apache 2.4, PHP 5.5.11 and systemd version 212-1 the config above will make the PHP error log end up somewhere similar to /tmp/systemd-private-65d4c3e3cc534155bcf3ec708c2362f6-httpd.service-h8MSsD/tmp/php_error_log.