MySQL добавление значений к переменной

<?php
include "db.php";
$SCORE = $_GET['score'];
$USERNAME = $_GET['username'];
$result = mysqli_query($connection,"SELECT score FROM users WHERE username = '$USERNAME'");
while ($row = $result->fetch_assoc())
{
    echo ($new = $row['score'] + 1);
}
mysql_query (" UPDATE users SET score=$new WHERE username='USERNAME'") ;
?>

<?php
include "db.php";
$SCORE = $_GET['score'];
$USERNAME = $_GET['username'];
$result = mysqli_query($connection,"SELECT score FROM users WHERE username = '$USERNAME'");
while ($row = $result->fetch_assoc())
{
  echo  $row['score'];
}
mysql_query (" UPDATE users SET score=score+1 WHERE username='USERNAME'") ;
?>

<?php
include "db.php";
$SCORE = $_GET['score'];
$USERNAME = $_GET['username'];
$result = mysqli_query($connection,"SELECT score FROM users WHERE username = '$USERNAME'");
while ($row = $result->fetch_assoc())
{
  echo  $row['score'];
}
$result2 = mysql_query (" UPDATE users SET 'score'+=1 WHERE username='USERNAME'") ;
if ($result2)
{
    echo "<p>ДА</p>";
}
else
{
    echo "<p>НЕТ</p>";
}
?>

<?php
include "db.php";
$SCORE = $_GET['score'];
$USERNAME = $_GET['username'];
$result = mysqli_query($connection,"SELECT score FROM users WHERE username = '$USERNAME'");
while ($row = $result->fetch_assoc())
{
  echo  $row['score'];
}
$result2 = mysqli_query($connection,"UPDATE users SET score+=1 WHERE username='$USERNAME'") or die ("НЕТ");
?>