There is a strange behaviour with json_encode and json_decode and I can't find a solution:

My php application calls a php web service. The webservice returns json that looks like this:

var_dump($foo):
string(62) "?{"action":"set","user":"123123123123","status":"OK"}"

now I like to decode the json in my application:

$data = json_decode($foo, true)

but it returns NULL:

var_dump($data):
NULL

I use php 5. The Content-Type of the response from the webservice: "text/html; charset=utf-8" (also tried to use "application/json; charset=utf-8")

What could be the reason?