update one specific row from an existing user by a webform - webforms

Question

i have try multiple ways but i need to update an specific row from an existing user by a webform i have try this code but i have some thing wrong on the code.
<?php
session_start();
include_once 'dbconnect.php';
if(!isset($_SESSION['user']))
{
header("Location: index.php");
}
$res=mysql_query("SELECT * FROM users WHERE user_id=".$_SESSION['user']);
$userRow=mysql_fetch_array($res);
while($row = mysqli_fetch_array($result)){
echo '<form action="" method="post">';
echo '<div style="float:left">';
echo '<table border="1" bordercolor="#000000">';
echo '<tr>';
echo '<td>link</td>';
echo '<td><input type="text" name="user_wallet" value="'.$row['user_wallet'].'"></td>';
echo '</tr>';
echo '</table></div>';
echo '</form><br /><br /><br /><br /><br /><br /><br /><br />';
}
}
if(isset($_POST['update'])){
$user_wallet = $_POST['user_wallet'];
$sql = mysqli_query("UPDATE users SET user_wallet = '$user_wallet' WHERE id = '$id'");
$retval = mysqli_query( $sql, $con );
if(! $retval ){
die('Could not update data: ' . mysql_error());
}
echo "Updated data successfully\n";
}
?>