Apache httpd process be killed by kernel oom

Personally I would never use PAE (over 4G of RAM on a 32-bit system). You'll get much better mileage running an actual 64-bit kernel and system.

OOM should only trigger when a malloc might fail. (not when you have lots of swap available)

The 32-bit kernel is likely to be part of the cause. PAE uses different memory zones and it may be that one zone isn't allowed to malloc from another.

Have you modified your swappiness? (How readily the kernel will use swap.) cat /proc/sys/vm/swappiness ?

You might also investigate tuning vm.dirty_ratio or vm.lower_zone_protection = 100.

Have you captured the kernel panic? (a serial console is often a good way to do this)

You can also try to preempt the OOM-Killer with your own process monitoring software. (take a look at Monit)

Best of luck

I hate to say it, but you appear to be asking the wrong question.

It's not about stopping Apache from bringing down your server, it's about having your webserver serve more queries per second - enough so that you don't have a problem. A part of the answer to the reframed question is then limiting Apache so that it does not crash at high loads.

For the second part of that, Apache has some limits you can set - MaxClients being an important configuration. This limits how many children it's allowed to run. If you can take load off Apache for long-running processes (large files being downloaded for example), that's another slot in Apache to be able to serve PHP. If the file downloads have to be verified by the PHP layer, they can still do that, and pass back out to a more optimised webserver for the static content, such as with NginX sendfile

Meanwhile, forking Apache every on every single request for the slowest way to run PHP - as a CGI (whatever apache MPM you may be using) - is also having the machine spend large amounts of time not running your code. mod_php is significantly more optimised.

PHP can do huge amounts of traffic when Apache and the PHP layer are appropriately optimised. Yesterday, 11th Dec 2010, for example, the pair of PHP servers that I run did almost 19 Million hits in the 24hr period, and most of that in the 7am-8pm time-period.

There are plenty of other questions here, and articles elsewhere about optimising Apache and PHP, I think you need to read them first, before blaming Linux/Apache & PHP.