Our php.log was nearing 550MB so I was investigating how to rotate the logs. The easiest solution seems to be using logrotate (ubuntu linux) with a script like follows.
sudo nano /etc/logrotate.d/php5
/var/log/php5/*.log {#rotate daily daily
#keep 14 days rotate 14
missingok nocompress sharedscripts postrotate apache2ctl graceful endscript
}
You can check the configuration by running logrotate --force /etc/logrotate.d/php5