PHP file_get_contents() returns "failed to open stream: HTTP request failed!"

Try using cURL.

<?php

$curl_handle=curl_init();
curl_setopt($curl_handle, CURLOPT_URL,'http://###.##.##.##/mp/get?mpsrc=http://mybucket.s3.amazonaws.com/11111.mpg&mpaction=convert format=flv');
curl_setopt($curl_handle, CURLOPT_CONNECTTIMEOUT, 2);
curl_setopt($curl_handle, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($curl_handle, CURLOPT_USERAGENT, 'Your application name');
$query = curl_exec($curl_handle);
curl_close($curl_handle);

?>

Could this be your problem?

Note: If you're opening a URI with special characters, such as spaces, you need to encode the URI with urlencode().