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

#php