How to Keep the selected value of the select box after Form POST or GET

Question

Im trying to implement the search feature in my website.

when the search keyword is entered in the textbox, and the category combo is selected, the form will be Pos

Answer 1

Assuming that by "combo" you mean "A regular select element rendering as a drop down menu or list box" and not "A combobox that is a combination of a drop down menu and free text input":

When outputting the elements, check the value against the submitted data in $_POST / $_GET and output selected (in HTML) or selected="selected" (in XHTML) as an attribute of the option element.