When developing you might just need to clear your Apache error log files from time to time. There is a brillant answer to this problem on Serverfault.

Assuming you are logging to the default error.log:

#Debian based: Ubuntu etc..
sudo su
echo > /var/log/apache2/error.log

#RHL based : CentOS, Fedora etc...
sudo su
echo > /var/log/httpd/error.log

That truncates the file without closing any open file handles. Note that you need to be root user, a simple sudo might still throw permission errors.