If the apache error log is showing something similar to this:

[Fri Nov 19 18:20:00 2004] [warn-phpd] mmap cache can’t open /home/httpd/vhosts/example.com/httpdocs/footer.php - Too many open files (pid 32133)
Traceback (most recent call last):
 

Apache is hitting it’s limit on the number of files it can open.  To view the current amount of files allowed to be opened, use this command: 

   ulimit -n

Edit the apache startup script, whether it be /etc/init.d/httpd, or /usr/local/apache/bin/apachectl, and add this before anything else:

# Raise The File Limit
ulimit -n 16384 
 
Specifying a value after the ‘-n’ flag will change the limit to that value.  When setting the value, remember that it must be in increments of 1024.  Now restart apache using one of the above referenced startup scripts and this should alleviate the problem.