Opciones de contexto HTTP

Note that if you set the protocol_version option to 1.1 and the server you are requesting from is configured to use keep-alive connections, the function (fopen, file_get_contents, etc.) will "be slow" and take a long time to complete. This is a feature of the HTTP 1.1 protocol you are unlikely to use with stream contexts in PHP.
Simply add a "Connection: close" header to the request to eliminate the keep-alive timeout:
<?php
// php 5.4 : array syntax and header option with array value
$data = file_get_contents('http://www.example.com/', null, stream_context_create([
 'http' => [
 'protocol_version' => 1.1,
 'header' => [
 'Connection: close',
 ],
 ],
]));
?>

note that for both http and https protocols require the same 'http' context keyword:
<?php
// CORRECT example:
// this will work as expected
// note the url with https but context with http
$correct_data = file_get_contents('https://example.com', false, stream_context_create(array('http' => array(...))));
// INVALID example:
// this will not work, the context will be ignored
// note the url with https also context with https
$correct_data = file_get_contents('https://example.com', false, stream_context_create(array('https' => array(...))));

With the default of 
 'follow_location' => 1
be certain NEVER include a "Host:" header in the 'header' array.
If the host is set to "mydomain.com", and that web site has a (quite common) redirect to "www.mydomain.com", then the initial request to "http://mydomain.com" will get the expected response of:
HTTP/1.1 301 Moved Permanently
Location: http://www.mydomain.com/
However, the follow-up request does NOT replace the original "host" header with the new "location" value, as one would expect. Consequently each "follow-location" request will again be served by the original host of "http://mydomain.com", and continue the redirect loop until 'max_redirects' has been exhausted.
(For details: https://bugs.php.net/bug.php?id=77889)

note that both http and https transports require the same context name http
// OK example:
// this will work as expected
// note the url with https but context with http
$correct_data = file_get_contents('https://example.com', false, stream_context_create(array('http' => array(...))));
// INVALID example:
// this will not work, the context will be ignored
// note the url with https also context with https
$correct_data = file_get_contents('https://example.com', false, stream_context_create(array('https' => array(...))));

watch your case when using methods (POST and GET)...it must be always uppercase. in case of you write it in lower case it wont work.

Note that setting request_fulluri to true will *change* the value of $_SERVER['REQUEST_URI] on the receiving end (from /abc.php to http://domain.com/abc.php).

If you want to use Basic Auth while using get_headers(), use stream_context options below. 
I am using HEAD method here, but please feel free to use GET also.
<?php
$targetUrl = 'http or https target url here';
$basicAuth = base64_encode('username:password');
stream_context_set_default(
 [
 'http' => [
 'method' => 'HEAD',
 'header' => 'Authorization: Basic ' . $basicAuth
 ]
 ]
);
$result = get_headers($targetUrl);
print_r($result);

If you use the proxy server and encounter an error "fopen(http://example.com): failed to open stream: HTTP request failed! HTTP/1.0 400 Bad Request" note that in many situations you need also set the parameter "request_fulluri" to "true" in your stream options. Without this option the php script sends the empty request to the server as "GET / HTTP/0.0" and the proxy server replies to it with the "HTTP 400" error. 
For example (working sample): 
<?php
$stream = stream_context_create(Array("http" => Array("method" => "GET",
 "timeout" => 20,
 "header" => "User-agent: Myagent",
 "proxy" => "tcp://my-proxy.localnet:3128",
 'request_fulluri' => True /* without this option we get an HTTP error! */
 )));
if ( $fp = fopen("http://example.com", 'r', false, $stream) ) {
 print "well done";
}
?>

P>S> PHP 5.3.17

It's worth noting that the `header` array seems to only want an array of strings, not an associative array.
I just spent a chunk of time debugging something not working as expected (no errors though) which was fixed by converting an associative array of headers into a simple array of strings.

Remember to match content with Content-type:
<?php
$data = array(
 'var1' => 'some content',
 'var2' => 'doh'
);
$opts = array('http' =>
 array(
 'method' => 'POST',
 'header' => 'Content-type: application/json', // here...
 'content' => json_encode($data) // and here.
 )
);
. . .
?>

I had quite a bit of trouble trying to make a request with fopen through a proxy to a secure url. I kept getting a 400 Bad Request back from the remote host. It was receiving the proxy url as the SNI host. In order to get around this I had to explicity set the SNI host to the domain I was trying to reach. It's apparently the issue outlined in this bug: 
https://bugs.php.net/bug.php?id=63519
<?php
$domain = parse_url($file, PHP_URL_HOST);
$proxy_string = "tcp://" . WP_PROXY_HOST . ":" . WP_PROXY_PORT;
$opts = array( 
 'http' => array( 'proxy' => $proxy_string ),
 'ssl' => array( 'SNI_enabled' => true, 'SNI_server_name' => $domain));
$context = stream_context_create($opts);
$handle = fopen( $file, 'r', false, $context );
?>

If you pass the 'header' option as a string and you have multiple headers, they must be separated by "\r\n".