Write all PHP errors, warnings and notices to error_log Drupal catches PHP warnings and notices to log them via dblog or syslog. As a result, there are two places to monitor for PHP errors (with the watchdog UI not well suited to cater for any large number of notices).

Is it possible to prevent this behavior and have ALL php errors be logged to the error_log defined in the php config?
For that, you will need to implement the hook_watchdog to make calls to the error_log() function.

You could as example one of the places where is implemented, that is the syslog_watchdog. Instead of use the openlog() and syslog() function, you will use the error_log().