Example

<?php
$url       = 'https://www.paypal.com/cgi-bin/webscr';
$tier      = $_GET["tier"];
$form_data = array(
    'business' => 'business@jerr0w.xyz',
    'notify_url' => 'https://donate.trinia.pro/classes/paypal-listener.php',
    'cancel_return' => 'https://donate.trinia.pro/verify_error.php',
    'return' => 'https://donate.trinia.pro/verify_success.php',
    'rm' => '2',
    'no_shipping' => '1',
    'no_note' => '1',
    'currency_code' => 'EUR',
    'page_style' => 'paypal',
    'charset' => 'utf-8',
    'cmd' => '_xclick'
);

// This would be the user's steamid, could just use https://github.com/SmItH197/SteamAuthentication
// since he's already using it, and wouldn't have to trust the clients steamid
$form_data["custom"] = "";

switch ($tier) {
    case 1:
        $form_data["item_name"] = "Donation: Tier I";
        $form_data["amount"]    = 0.01;
        break;
    case 2:
        $form_data["item_name"] = "Donation: Tier II";
        $form_data["amount"]    = 10.00;
        break;
    case 3:
        $form_data["item_name"] = "Donation: Tier III";
        $form_data["amount"]    = 15.00;
        break;
    case 4:
        $form_data["item_name"] = "Donation: Tier IV";
        $form_data["amount"]    = 25.00;
        break;
    case 5:
        $form_data["item_name"] = "Donation: Tier V";
        $form_data["amount"]    = 50.00;
        break;
    default:
        die("Invalid tier");
}

$options = array(
    'http' => array(
        'header' => "Content-type: application/x-www-form-urlencoded\r\n",
        'method' => 'POST',
        'content' => http_build_query($form_data)
    )
);
$context = stream_context_create($options);
$result  = file_get_contents($url, false, $context);
$headers = parseHeaders($http_response_header);

if (strpos($headers[0], '302') !== false) {
    header("Location: " . $headers["Location"]);
}

//Taken from http://php.net/manual/en/reserved.variables.httpresponseheader.php#117203
function parseHeaders( $headers )
{
    $head = array();
    foreach( $headers as $k=>$v )
    {
        $t = explode( ':', $v, 2 );
        if( isset( $t[1] ) )
            $head[ trim($t[0]) ] = trim( $t[1] );
        else
        {
            $head[] = $v;
            if( preg_match( "#HTTP/[0-9\.]+\s+([0-9]+)#",$v, $out ) )
                $head['reponse_code'] = intval($out[1]);
        }
    }
    return $head;
}
?>