An error similar to this appears in the Apache error log:
Any version of Zend Server for IBM i running on any supported version of IBM i.
Apache 3425 errors indicate a problem receiving a response from the end user browser. This can happen if, for example, the user navigates away from the page while it is still downloading, often because the page contains a large graphic or PDF, or some other large content, or runs a long running query. It might also happen if the page stops downloading for a time because some file needed from the IFS is locked. The browser may time out waiting for the content. It may also happen if the page has a form button that the user can click repeatedly if they become impatient waiting for the page to load.
This message is usually treated as a warning. The user will try the page again and hopefully wait for the content. To reduce the occurrences of these messages, check your Events for long running scripts and work on making them run faster. For example, if there is a long running query, consider reworking the query to only pull the needed records, and make sure there is a Logical FIle with an access path that can help speed up the query. For large file downloads, see if you can compress the file for faster downloading.
Also, if your forms have an entry button that can be clicked multiple times, please disable the button after the first click. There are a number of ways to do this. Search Google for the phrase 'disable duplicate form submission' to see many examples. If you see a lot of php timeout errors on the session_start() function, this could be your issue.
Learn more about how to prevent session_start() timeouts
The following article tells much more about how to properly manage sessions in your PHP scripts