Someone

 echo "<td>" . $row['Username'] . "</td>";

  $lp = Lap_name;
  if ($lp=="Lap_name") 
  {
  echo "<td>" . $row['$Lap_name'] . "</td>"; 
           }elseif{

  ($lp=="Pc_name") 
  
  echo "<td>" . $row['Pc_name'] . "</td>";
  
             else echo "N/A";

  echo "<td>" . $row['ip_adresse'] . "</td>";