Note that the HTTP wrapper has a hard limit of 1024 characters for the header lines.
Any HTTP header received that is longer than this will be ignored and won't appear in $http_response_header.

The cURL extension doesn't have this limit.

http_fopen_wrapper.c: #define HTTP_HEADER_BLOCK_SIZE 1024

parser function to get formatted headers (with response code)

<?php

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;
}

print_r(parseHeaders($http_response_header));

/*
Array
(
    [0] => HTTP/1.1 200 OK
    [reponse_code] => 200
    [Date] => Fri, 01 May 2015 12:56:09 GMT
    [Server] => Apache
    [X-Powered-By] => PHP/5.3.3-7+squeeze18
    [Set-Cookie] => PHPSESSID=ng25jekmlipl1smfscq7copdl3; path=/
    [Expires] => Thu, 19 Nov 1981 08:52:00 GMT
    [Cache-Control] => no-store, no-cache, must-revalidate, post-check=0, pre-check=0
    [Pragma] => no-cache
    [Vary] => Accept-Encoding
    [Content-Length] => 872
    [Connection] => close
    [Content-Type] => text/html
)
*/

?>

This feature has been deprecated as of PHP 8.5.0.
Here is a way to use it and be compatible with old and modern PHP version:

<?php
function get_contents() {
  file_get_contents("http://example.com");

  // For PHP >= 8.4.0
  if (function_exists('http_get_last_response_headers')) {
    $http_response_header = http_get_last_response_headers();
  }

  // No need for older PHP version
  var_dump($http_response_header);
}
?>