Blank white pages when submitting forms in Drupal

This has bitten me enough that I'm going to write a blog post about it so when I search for this issue, I will find my own blog post and say, "Duh!"

Recently we had one site in development that presented a white screen whenever a form was submitted. Other sites right next to it worked fine. The form was being processed. But no errors were appearing either on the screen or in the error log for Apache or PHP -- all we saw was a blank page.

Then I noticed that the output_buffering setting in php.ini was set to Off. I turned it on and it solved the problem. But I wanted to know why it solved the problem. Here's why, and why it doesn't really solve the problem.

When Drupal's form handler completes processing of a form submission, it normally does a 302 redirect. The code responsible for that is in drupal_goto() and looks like this:

Running the code through a debugger showed that indeed, we were reaching the exit() call.

If any of your code has sent anything back to the browser, PHP has already sent the headers to the browser. If output_buffering is Off, Drupal cannot take that back and say no, really we meant to send you a 302. But if output buffering is on, the headers are not really sent, so calling header() still works. That's why turning on output buffering solves the problem.

But that's just treating the symptom. The real problem is that somewhere in your code, something is being sent back to the browser when it shouldn't be. This is the problem being referred to when the Drupal coding standards say, Note that the final ?> should be omitted from all code files--modules, includes, etc. The closing delimiter is optional, and removing it helps prevent unwanted white space at the end of files which can cause problems elsewhere in the system.

In my case, a designer had put a drupal_add_js(..., 'inline') call directly into template.php. Moving the JavaScript to a .js file solved the real problem.