Custom form in PHP goes to search page

Hi,

I've searched the forums etc, but although I found a number of similar-sounding questions, none of the suggested fixes worked for me.

I have a custom straight-HTML form created via PHP in a Page block.

The form action should POST back to the same page, so it can do PHP validation. Yes, I gather that the forms module will help with this kind of thing, but for various reasons I'm not using that.

The form is declared as:

Though I've also tried lots of other actions, or removing the action entirely, to no avail. In most situations, submitting the form starts loading the correct page, then loads the "search/node/" page instead of the current page. Why is that? Does the search module hijack the POST data? How can I make it load the current page again?