<?php

$user = 'root';//Seu usuário do banco de dados
$pass = '???';//Seu senha de conexão.
$conn = new PDO('mysql:host=localhost;dbname=test', $user, $pass);

$destUrl = 'http://steamcommunity.com/profiles/' . $steamid . '/inventory/json/730/2/';

$data = file_get_contents($destUrl, false);
$data = json_decode($data, true);
$data1= array_keys($data['rgDescriptions']);
$data2= $data['rgDescriptions'];

$conn->beginTransaction();

$sql = 'INSERT INTO sua_tabela (id, name, market_hash_name, market_name)
    VALUES (?, ?, ?, ?)';

$conn = $dbh->prepare($sql);

for($i = 0; $i < count($data1); ++$i) {

    $conn->execute(array(
        $data1[$i],
        $data2[$data1[$i]]['name'],
        $data2[$data1[$i]]['market_hash_name'],
        $data2[$data1[$i]]['market_name'];
    ));

}

$dbh->commit();