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

$countries_uid = $_POST['countries_uid'];
                while($row = mysql_fetch_array($result)){
                  $uid = $row['uid'];
                  $country = $row['country_name'];
                  $isSelected = null;
                    if(!empty($countries_uid)){
                    foreach($countries_uid as $country_uid){//cycle through country_uid
                       if($row['uid'] == $country_uid){
                        $isSelected = 'selected="selected"'; // if the option submited in form is as same as this row we add the selected
                       }
                    }
                   }else {
                    $isSelected = ''; // else we remove any tag
                  }
                  echo "".$country."";
                }

this is my solutions of multiple select dropdown box after modifying Mihai Iorga codes