mb_parse_str not working

php > echo "FORMDATA:\n\n$a\n\n/FORMDATA";
FORMDATA:

------------------------------b2449e94a11c
Content-Disposition: form-data; name="user_id"

3
------------------------------b2449e94a11c
Content-Disposition: form-data; name="post_id"

5

/FORMDATAphp > mb_parse_str($a,$out);
php > print_r($out)
php > ;
Array
(
    [------------------------------b2449e94a11c
Content-Disposition:_form-data;_name] => "user_id"

3
------------------------------b2449e94a11c
Content-Disposition: form-data; name="post_id"

5
)
php >