https://support.managed.com/kb/a1937/how-to-enable-php-error-logging-in-plesk-11.aspx
How to Enable PHP Error Logging in Plesk 11
This is for Plesk 11 only and will place a php.log.resources file in the httpdocs folder that will contain the PHP error log. If you have a dedicated server, you can perform this yourself. If you have a shared account it will need to be done by a support technician.
error_log = "{DOCROOT}\php.log.resources"
8. Click OK.