Query to Update Username/Password

<?php

   // Necessary include files:

   include "db.php";

   include "functions.php";

?>


<?php

   if ( isset($_POST['submit']) ) {

      $username = $_POST['username'];
      $password = $_POST['password'];
      $id = $_POST['id'];

      // Build query:

      $query =    "UPDATE users SET ";
      $query .=   "username = '$username', ";
      $query .=   "password = '$password' ";
      $query .=   "WHERE id = $id";

      // Perform query.

      $result = mysqli_query($connection, $query);

      // See if query was successful.

      if (!$result) {

         exit("QUERY FAILED: " . mysqli_error($connection) );

      }  // end if

   }  // end if

?>


<!DOCTYPE html>

<html lang="en">

   <head>

      <meta charset="utf-8" />

      <title>Login</title>

      <link rel="stylesheet"
       href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css"
       integrity="sha384-1q8mTJOASx8j1Au+a5WDVnPi2lkFfwwEAa8hDDdjZlpLegxhjVME1fgjWPGmkzs7"
       crossorigin="anonymous" />

   </head>

   <body>

      <div class="container">

         <div class="col-sm-6">

            <form action="login_update.php" method="post">

               <div class="form-group">

                  <label for="username">Username</label>

                  <input type="text" name="username" class="form-control" />

               </div> <!-- END form-group -->

               <div class="form-group">

                  <label for="password">Password</label>

                  <input type="password" name="password" class="form-control" />

               </div> <!--END form-group -->

               <div class="form-group">

                  <select name="id">

                     <?php

                        // Get all user ids for select options.

                        showAllData();

                     ?>


                  </select>

               </div> <!-- END form-group -->

               <input class="btn btn-primary" type="submit"
                name="submit" value="UPDATE" />

            </form> <!-- END form -->

         </div> <!-- END div-col-sm-6 -->

      </div> <!-- END container -->

   </body>

</html>