Php – 503 service unavailable debugging PHP files

You could try changing to a different MPM, which you can do in the configuration, or tweaking the parameters of your MPM. For example, if you're using the prefork or worker MPMs then you can increase the MaxClients value. In the latter case, ThreadsPerChild might also be a useful configuration value to tweak.

You can find information on the MPMs in the standard Apache documentation.

However, unless you're running a really busy website, I'm surprised you're seeing 503 errors at all - is it possible that there's some really slow server-side code that's tying up Apache requests for far too long? If so, you might get more mileage improving that than just trying to work around it in Apache.

Can you paste in what the Apache logs say in one instance where an image is not displayed? The first thing I'd like to know is if Apache itself sees the image-dropping as a problem.

As for the 503...

HTTP 503 is the standard HTTP error which just means Service Unavailable as you probably already know; It could be that a program (required program) has not started on the web-server, or has otherwise crashed.

This one should make a clear entry in the logs of the error of what exactly went wrong - because the web server is obviously sending the 503.

I think the 503 should be easier to troubleshoot than the first (missing images) problem, and may leak more info to solve the first problem too.

Good luck!