Login button not showing after upgrade to PHP 5.3 I upgraded our web server to PHP 5.3 as a bit of software requires it, and suddenly the login button on one of our Drupal websites has disappeared! Submit buttons from other Drupal forms have disappeared too.

I've looked in the source code of the page, and the form is indeed missing a submit button.

Is this a known problem? If not, how might I diagnose the error?
To diagnose the problem, make sure you display all errors (notices included) and that they reported somewhere (log, watchdog, ...), it should be a function incompatibility. Usually it is about parameters that are passed by reference for certain native functions or something like that (introduced in PHP5.3).

Then when you have identified the problem, open an issue on the module project (I bet it will be a contrib module or one of your custom module) so that the module maintainer can update it.

Drupal 6 is relatively old and versions prior to 6.14 are not working with PHP 5.3. Make sure you have the latest version, see also http://drupal.org/requirements/#php.