<?php
$con=mysqli_connect("127.0.0.1","root","","forms");
if (mysqli_connect_errno())
  {
  echo "Failed to connect to MySQL: " . mysqli_connect_error();
  }

  $sql=mysqli_query($con,"SELECT * FROM register Where username='$_POST[username]' AND password='$_POST[password]'");

  if (!mysqli_query($con,$sql))
  {
  die('Error: ' . mysqli_error($con));
  }
echo "login success";

mysqli_close($con);
?>

if(!$sql)
{
  //do something
}

if (!mysqli_query($con,$sql))

if($sql === FALSE)

$sql="SELECT * FROM register Where username='".$_POST[username]."' AND password='."$_POST[password]."'";

  if (!mysqli_query($con,$sql))
  {
  die('Error: ' . mysqli_error($con));
  }

<?php
      $con=mysqli_connect("127.0.0.1","root","","forms");
if (mysqli_connect_errno())
  {
  echo "Failed to connect to MySQL: " . mysqli_connect_error();
  }

  $sql="SELECT * FROM register Where username='$_POST[username]' AND password='$_POST[password]'";

  if (!mysqli_query($con,$sql))
  {
  die('Error: ' . mysqli_error($con));
  }
echo "login success";

mysqli_close($con);
?>

$sql = 'SELECT * FROM register Where username='' . $_POST[username]. '' AND password=''. $_POST[password].'' ';
$response=mysqli_query($con, $sql);
if (empty( $response ))
{
    die('Error: ' . mysqli_error($con));
}

if (!mysqli_query($con,$query))
    {
$query = mysqli_query($con, "SELECT * FROM combo WHERE combo_id='$cid'");
    $row=mysqli_fetch_array($query);
        $price=$row['combo_price'];
        $payable=$pax*$price;

mysqli_query($con,"UPDATE reservation SET payable='$payable',balance='$payable',r_venue='$venue',r_date='$date',r_time='$time',r_motif='$motif'
,r_ocassion='$ocassion',r_type='$type',pax='$pax',combo_id='$cid',price='$price' where rid='$id'")or die(mysqli_error($con));

    $_SESSION['id']=$id;