<?php

//$json = '[{"sku":"5200001200057","model":"516187A"},{"sku":"5200001200002","model":"521387"}]'; // work typed manually as prospekt.json 
$json   = '[{"sku":"5200001200057","model":"516187A"},{"sku":"5200001200002","model":"521387"}]'; // prospekt.json not work 

$res = json_decode($json, true); // возвращает NULL

print_r($res);