<?php
$db = new mysqli($host, $user, $pass, $dbname);
$orderArr = json_decode($_POST, true);
$sqlVals = array();
foreach ($orderArr as $i => $val) {
    $sqlVals[] = "'{$val}' = '{$i+1}'";
}
$quer = "UPDATE `table_name` SET " . implode(',', $sqlVals) . " WHERE id = 1";
$db->query($quer);