如何在多个选择中保留所选值，如bootstrap-select

Question

I want to retain the value that I selected after form submission.

<select name="student[]" class="selectpicker" multiple title="Click here.." data-width="auto" data-live-search="true" required>
  <?php  
    if (isset($_POST['submit'])){
    $selected = $_POST['studname'];
    $qry = "Select studtbl.stud_id,concat(studtbl.fname,' ', 
    substring(studtbl.mname, 1,1),'. ',studtbl.lname) as Name from studtbl";
        $result = mysqli_query($conn, $qry);
        while($row = mysqli_fetch_array($result))
        {
        extract($row);
        echo '<option value="'.$stud_id.'" '.(($stud_id == 
        $selected)? 'selected="selected"':"" >'.$Name.'</option>';
        } 
    }else{
    $qry = "Select studtbl.stud_id,concat(studtbl.fname,' ', 
    substring(studtbl.mname, 1,1),'. ',studtbl.lname) as Name from studtbl";
                $result = mysqli_query($conn, $qry);
                while($row = mysqli_fetch_array($result))
                {
                    extract($row);
                    echo '<option value="'.$stud_id.'" >'.$Name.'</option>';
                } 
    }
    ?>
</select>
<input type='submit' class="btn btn-primary" name='submit' value='Create 
     Account' />

My problem is it is not retaining after submitting

</div>