Getting an uncaught exception in PHP when trying to get data from POST API

I'm trying to access a list of most recent comments on Stack Overflow using this PHP:

<?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;
    }
    echo do_post_request("https://api.stackexchange.com/2.1/comments?order=desc&sort=creation&site=stackoverflow", "");
?>

However, when I run it, I get this error message:

$ php index.php 

PHP Notice:  Undefined variable: php_errormsg in /var/www/secomments/index.php on line 14
PHP Fatal error:  Uncaught exception 'Exception' with message 'Problem with https://api.stackexchange.com/2.1/comments?order=desc&sort=creation&site=stackoverflow, ' in /var/www/secomments/index.php:14
Stack trace:
#0 /var/www/secomments/index.php(22): do_post_request('https://api.sta...', '')
#1 {main}
  thrown in /var/www/secomments/index.php on line 14

Does anyone have any ideas as to what might be causing this, and what one would do to fix it?

The API, in this situation, should be called from the method get.

If you do visit the API at this link: https://api.stackexchange.com/2.1/comments?order=desc&sort=creation&site=stackoverflow
You are presented with a nice JSON with all the information you want.

If instead, you fix up the post params:

$params = array('http' => array(
    'method' => 'POST',
    'content' => $data,
    'header' => 'Content-Length: ' . strlen($data)
));

You are shown this instead:

{"error_id":404,"error_name":"no_method","error_message":"this method cannot be called this way"}

Hopefully you know that you can simply just use file_get_contents to contact the API with the traditional get.

$json = json_decode(file_get_contents("https://api.stackexchange.com/2.1/comments?order=desc&sort=creation&site=stackoverflow"), true);