Why does the Apache server keep stopping

A packet trace from the httpd server/LDAP client revealed a message about the CA being unknown.

TLSv1 Alert (Level: Fatal, Description: Unknown CA)

I found and added the following option to my httpd.conf:

  LDAPVerifyServerCert          off

That fixed my issue under CentOS 6. The CentOS 5 httpd servers did not require any modification and had worked all along without the option.

The best method for reducing apache's memory usage is to move away from mod_php and to something like fastcgi. Each of your apache processes is 15mb or more because of mod_php overhead (most likely). Having php requests handed off to fastcgi will reduce the average apache process size to approx 1mb or so depending on the apache configuration.

Since php is now centralized using fastcgi it's memory usage is more efficient and the total amount of memory used by the system should decrease slightly.

Another approach would be to place an http server that's more memory efficient in front of apache and have it server static content directly and proxy non-static requests to apache. Nginx would be great for this.

As a temporary fix you can also look into decreasing the MaxRequestsPerChild to 1000 or something more aggressive. Since apache processes tend to grow in size as they serve requests this will limit their size by killing them off and spawning new ones.