Where does PHP-FPM and PHP-CLI store the error log?

Question

When running PHP-FPM or PHP cli if I uncomment the error_log directive - where does PHP store this file?

; Log errors to specified file. PHP's default behavior is to leave this value
; empty.
; http://php.net/error-log
; Example:
error_log = php_errors.log

I'm asking how /etc/php5/cli/php.ini and /etc/php5/fpm/php.ini figure out the relative error_log path.

Note: This has nothing to do with Apache[1-2]