Google Short URL API: Forbidden - php

Question

I have what I think is correctly written code yet whenever I try and call it I'm getting permission denied from Google.
file_get_contents(https://www.googleapis.com/urlshortener/v1/url): failed to open stream: HTTP request failed! HTTP/1.0 403 Forbidden
This isn't a rate limit or anything as I currently have zero ever used...
I would have thought this is due to an incorrect API key but I've tried resetting it a number of times. There isn't some downtime while the API is first applied is there?
Or am I missing a header setting or something else just as small?
public function getShortUrl()
{
$longUrl = "http://example.com/";
$apiKey = "MY REAL KEY IS HERE";
$opts = array(
'http' =>
array(
'method' => 'POST',
'header' => "Content-type: application/json",
'content' => json_encode(array(
'longUrl' => $longUrl,
'key' => $apiKey
))
)
);
$context = stream_context_create($opts);
$result = file_get_contents("https://www.googleapis.com/urlshortener/v1/url", false, $context);
//decode the returned JSON object
return json_decode($result, true);
}

Answer 1

It seems I need to manually specify the key in the URL
$result = file_get_contents("https://www.googleapis.com/urlshortener/v1/url?key=" . $apiKey, false, $context);
This now works. There must be something funny with how the API inspects POST for the key (or lack of doing so).
Edit: For anyone in the future this is my complete function
public static function getShortUrl($link = "http://example.com")
{
define("API_BASE_URL", "https://www.googleapis.com/urlshortener/v1/url?");
define("API_KEY", "PUT YOUR KEY HERE");
// Used for file_get_contents
$fileOpts = array(
'key' => API_KEY,
'fields' => 'id' // We want ONLY the short URL
);
// Used for stream_context_create
$streamOpts = array(
'http' =>
array(
'method' => 'POST',
'header' => [
"Content-type: application/json",
],
'content' => json_encode(array(
'longUrl' => $link,
))
)
);
$context = stream_context_create($streamOpts);
$result = file_get_contents(API_BASE_URL . http_build_query($fileOpts), false, $context);
return json_decode($result, false)->id;
}