The title says it all: you can use an empty action attribute on a form to submit that form to the current page. This means you don't need to use server-side scripting (using REQUEST_URI or PHP_SELF or whatnot) to write the current URL into the HTML.
The following is perfectly valid:
<form action="" method="post">
Now beware, the action attribute is mandatory, and it must contain a valid URI. But according to the URI RFC, an empty URI is still a URI:
4.2. Same-document References
A URI reference that does not contain a URI is a reference to the current document. In other words, an empty URI reference within a document is interpreted as a reference to the start of that document, and a reference containing only a fragment identifier is a reference to the identified fragment of that document. Traversal of such a reference should not result in an additional retrieval action. However, if the URI reference occurs in a context that is always intended to result in a new request, as in the case of HTML's FORM element, then an empty URI reference represents the base URI of the current document and should be replaced by that URI when transformed into a request.
So there you have it. Enjoy.