$sql_ins = "INSERT INTO user_links (username, user_id, link_id, checked, last_clicked)
VALUES ('$username', '$u_id', '$id', 'yes', '$date_time')";

if (mysqli_query($conn, $sql_ins)) {

if ( $thistime - $date_time >=24) {
   $sqlt_ins=mysql_query("then insert new user_links");
}
else {echo "something error";}

} else {
  header('Location: ad-view.php?viewed');
}