in the example code, there is a form element named keywords and the search form is posting to the results page using the POST method.
the idea is to use an if statement to see if the keywords form element in the search form is blank:
if (isset($_POST['keywords"]) && $_POST['keywords"] == "") {
if it is left blank in the search form, then set the value to a nonsensical value that wouldn't appear in your database:
$_POST['keywords"] = "zznoresultszz";
this will cause the search to look for a value that doesn't exist, thus returning no records.