PHP: Submit a POST query and get RESPONSE

Question

What I need to do, is to make a script, which submit a $_POST query to remote host, and that host is returning an answer on $_POST:

<?php
if($_POST['passkey'] == "c")
   echo "OK";
else
   echo "FAILED";
?>

I use this code:

function get_client_answer( $source_host='http://www.example.com/',$source_file='validate.php') {
    $postdata = http_build_query(
        array(
            'message' => 'a',
            'amount' => 'b',
            'passkey' => 'c'
        )
    );

    $opts = array('http' =>
        array(
            'method'  => 'POST',
            'header'  => 'Content-type: application/x-www-form-urlencoded',
            'content' => $postdata,
            'max_redirects' => 0,
            'ignore_errors' => true,
            'timeout' => 10 // seconds
        )
    );
    $context = stream_context_create($opts);

    // Open the file using the HTTP headers set above
    $file_content = @file_get_contents($source_host.$source_file, false, $context);
    return $file_content;
}

But If I do:

<?php
echo get_client_answer("http://www.mywesite.com/", "validate.php");
?>

It returns me a blank text.

The problem is - how to get a response, that my last code will return "OK" or "FAILED"