PHP - Php Not Fully Running

if ($searchtype == "store_name")
   {
       $sql = "SELECT * FROM `shoplist` WHERE store_name LIKE :searchterm ORDER BY store_name ASC";
   }
else if ($searchtype == "item_description")
   {
       $sql = "SELECT * FROM `shoplist` WHERE item_description LIKE :searchterm ORDER BY store_name ASC";
   }

// Tried this first but it didn't work...gave some kind of error pertaining to the parameter 2    
// Prepare the query ONCE
$query = "SELECT * FROM `shoplist` WHERE ? LIKE ? ORDER BY ? ASC";

$searchterm = '%' . $searchterm . '%';

$statement = $conn->prepare($query);
$statement->bindValue(1, $searchtype);                                                        
$statement->bindParam(2, $searchterm);
$statement->bindValue(3, $searchtype);
$statement->execute();

// ----------------------------------------------------------------------------------------------------------- //

// Tried this as well, but was getting unexpected results (mostly all rows being returned, regardless of input)
// Prepare the query ONCE
$query = "SELECT * FROM `shoplist` WHERE :searchtype LIKE :searchterm ORDER BY :searchtype ASC";

$searchterm = '%' . $searchterm . '%';

$statement = $conn->prepare($query);
$statement->bindValue(':searchtype', $searchtype);
$statement->bindValue(':searchterm', '%' . $searchterm . '%');
$statement->execute();

// create short variable names
if(isset($_POST['searchtype']))
    {                                            
        $searchtype=$_POST['searchtype'];
    }
if(isset($_POST['searchterm']))
    {                            
        $searchterm=trim($_POST['searchterm']);
    }

if(isset($_POST['searchtype']) && isset($_POST['searchterm']))
    {
        if (!get_magic_quotes_gpc())
            {
                $searchtype = addslashes($searchtype);
                $searchterm = addslashes($searchterm);
            }
    }

if(isset($_POST['searchtype']) && isset($_POST['searchterm']))
    {
        // ****** Setup customized query ******
        if ($searchtype == "store_name")
            {
                // The following statement is potentially open to SQL-inection attack and has therefore been REM(arked)
                // $sql = "SELECT * FROM `shoplist` WHERE " . $searchtype . " LIKE :searchterm ORDER BY store_name ASC";

                // The following may not be open to SQL-injection attack, but has the column name hard-coded in-between the WHERE and LIKE clauses
                // I would like the the column name to be set based on the value chosen by the user in the form dropdown listbox for "searchtype"
                $sql = "SELECT * FROM `shoplist` WHERE store_name LIKE :searchterm ORDER BY store_name ASC";
            }
        else if ($searchtype == "item_description")
            {
                // The following statement is potentially open to SQL-inection attack and has therefore been REM(arked)
                // $sql = "SELECT * FROM `shoplist` WHERE " . $searchtype . " LIKE :searchterm ORDER BY item_description ASC";

                // The following may not be open to SQL-injection attack, but has the column name hard-coded in-between the WHERE and LIKE clauses
                // I would like the the column name to be set based on the value chosen by the user in the form dropdown listbox for "searchtype"
                $sql = "SELECT * FROM `shoplist` WHERE item_description LIKE :searchterm ORDER BY item_description ASC";
            }

        $statement = $conn->prepare($sql);
        $statement->bindValue(':searchterm', '%' . $searchterm . '%');
        $statement->execute();
        ...
        ...
        ...
    }