You can just echo, after your insert query.
if (isset($_POST['submitContent']))
{
$eid = $_SESSION['employeeId'];
$contentCategory = $_POST['contentCategory'];
$contentTitle = $_POST['contentTitle'];
$contentWriter = $_POST['writer'];
$displayMode = $_POST['status'];
$query = mysqli_query($con,"INSERT INTO contentdetails(employeeId,contentCategory,contentTitle,contentWriter,photo,displayMode,published_on)
VALUES('$eid','$contentCategory','$contentTitle','$contentWriter',
'$file_name','$displayMode',NOW())");
echo " ".$_GET["contentCategory"]." ".$_GET["contentTitle"]." , You Have Successfully Inserted !!";
}