apache 2.2 - What are some reasons why PHP would not log errors?

I'm running Apache2 on Ubuntu 10.04 LTS. I've configured /etc/php5/apache2/php.ini with the following:

error_reporting = E_ALL & ~E_DEPRECATED
display_errors = Off

log_errors = On
error_log = /var/log/php_errors.log

I have a PHP application that I believe has an error in it. With those settings and the suspected error, no php_errors.log was created. So, I created a PHP file with syntax errors. Still no php_errors.log. What are some reasons why PHP would not log errors?

Could it be permissions? I see that /var/log is owned by root:root. I don't know what owns the php process. I believe root starts the apache process and then a new starts a new thread using www-data:www-data for serving files. Would that mean I need to change permissions on the /var/log folder?