when does json_decode unable to decode json string as object?

PHP:

json_decode($userinfo);

will decode the json string as object

PHP:

json_decode($userinfo, true);

will decode the json string as array..
For some reason I'm able to extract the json string as array but when I try it to do it as object it breaks. Do you have any idea why?
Thanks

See original post by ameerulislam43