How to convert an associative array into a querystring?

UPDATE 1:

I also have the following foreach loop in my code:

foreach ($_POST as $key => $value){ 
    $message .= $key . " = " .$value ."\n\n<br />"; 
}

Which is the final one.

Or maybe the problem is in the line:

curl_setopt($ch, CURLOPT_POSTFIELDS, $req);

Doesn't like the %5B0%5D in the string generated, i.e.

transaction%5B0%5D=USD+21.00

using the code $req = http_build_query($_POST);?

ORIGINAL QUESTION:

I'm getting the following error message:

[Sun Aug 04 10:00:00 2012] [error] [client 000.0.00.000] PHP Warning:  stripslashes() expects parameter 1 to be string, array given in /var/www/page.php on line 20
[Sun Aug 04 10:00:00 2012] [error] [client 000.0.00.000] PHP Stack trace:
[Sun Aug 04 10:00:00 20122] [error] [client 000.0.00.000] PHP   1. {main}() /var/www/page.php:0
[Sun Aug 04 10:00:00 2012] [error] [client 000.0.00.000] PHP   2. stripslashes() /var/www/page.php:20

The problem code is this:

Line 19: foreach ($_POST as $key => $value) {
Line 20:     $value = urlencode(stripslashes($value));
Line 21:     $req .= "&$key=$value";
Line 22: }

Where $_POST contains the following:

Array
(
    [transaction] => Array
        (
            [0] => USD 21.00
        )

    [payment_request_date] => Sat Aug 04 10:00:00 PDT 2012
    [return_url] => http://000.000.000.000/success.php
    [fees_payer] => EACHRECEIVER
    [ipn_notification_url] => http://000.000.000.000/ipn.php
    [sender_email] => buyer_0394820394_per@gmail.com
    [verify_sign] => slkdfj30r2489urj0947830ru0ty80ur0er037jfgoiru932e.as329e
    [test_ipn] => 1
    [cancel_url] => http://000.000.000.000/cancel.php
    [pay_key] => AP-SLKJ30F9J90J3RSD3
    [action_type] => PAY
    [transaction_type] => Adaptive Payment PAY
    [tracking_id] => E4902RJF2
    [status] => COMPLETED
    [log_default_shipping_address_in_transaction] => false
    [charset] => windows-1252
    [notify_version] => UNVERSIONED
    [reverse_all_parallel_payments_on_error] => false
)

Is the stripslashes() giving me a problem because of the array within an array? If yes, how can I deal with the $_POST in the foreeach loop, if no, what could the problem be?

Just use the function http_build_query, which will generate a URL-encoded query string for you.

$query_str = http_build_query($_POST);