I don't think we do a very good job of evangelizing some of the nice things that the PHP streams layer does in the PHP manual, or even in general. At least, every time I search for the code snippet that allows you to do an HTTP POST request, I don't find it in the manual and resort to reading the source. (You can find it if you search for "HTTP wrapper" in the online documentation, but that's not really what you think you're searching for when you're looking).

So, here's an example of how to send a POST request with straight up PHP, no cURL:

<?php
  function do_post_request($url, $data, $optional_headers = null)
  {
     $params = array('http' => array(
                  'method' => 'POST',
                  'content' => $data
               ));
     if ($optional_headers !== null) {
        $params['http']['header'] = $optional_headers;
     }
     $ctx = stream_context_create($params);
     $fp = @fopen($url, 'rb', false, $ctx);
     if (!$fp) {
        throw new Exception("Problem with $url, $php_errormsg");
     }
     $response = @stream_get_contents($fp);
     if ($response === false) {
        throw new Exception("Problem reading data from $url, $php_errormsg");
     }
     return $response;
  }

$optional_headers is a string containing additional HTTP headers that you would like to send in your request.

PHP's HTTP wrapper will automatically fill out the Content-Length header based on the length of the $data that you pass in. It will also automatically set the Content-Type to application/x-www-form-urlencoded if you don't specify one in the $optional_headers.

I find this very handy; I don't need to code in redirection logic, HTTP auth handling, user agent setting and so on; they are handled for me by PHP.

You may also want to look into http_build_query() which is a convenience function that allows you to assemble query/post parameters from a PHP variable, applying appropriate escaping.

Kudos to Sara Golemon for both of these things. You can find more documentation on the HTTP wrapper options in the HTTP and HTTPS page in the PHP manual.