error: Notice: Undefined index: fk

Question

can someone assist me with fixing the error i'm receiving.

$search_key = '';
/*if(isset($_POST) && ($_POST["fk"])!=NULL){
    $search_key = $_POST["fk"];
}elseif(isset($_GET) && ($_GET["fk"]!=NULL)){
    $search_key = $_GET["fk"];
}
*/

if(!isset($_POST["fk"])){
    $search_key = $_POST['fk'];
}elseif(isset($_GET["fk"])){
    $search_key = $_GET["fk"];
}