<?php
$host = "localhost";
$username = "root";
$password = "ascent";
$database = "characters";

$oldCharacter = $_POST['oldCharacter'];
$newCharacter = $_POST['newCharacter'];

$con = new mysqli($host, $username, $password, $database);
$sql = "UPDATE character_inventory SET guid = ? WHERE guid = ?";

$stmt = $con->prepare($sql);
$stmt->bind_param('ii',$newCharacter,$oldCharacter);
$stmt->execute();
if ($stmt->execute()) {
   // it worked
$stmt->close();
} else {
   // it didn't
die('error: ' . mysqli_error($con));
}
?>