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