PHP BrowserID verification function

browserid.php

<?php

/**
 * Verifies the given BrowserID assertion
 *
 * @throws \UnexpectedValueException on error
 *
 * @param  string $assertion Assertion provided by navigator.id.getVerifiedEmail()
 * @param  string $audience  Host Name, defaults to $_SERVER['HTTP_HOST']
 * @param  string $verifyService URL of the verification webservice, defaults BrowserID's
 * @return object The decoded JSON returned by the verify service
 */
function browserid_verify_assertion($assertion, $audience = null, $verifyService = 'https://browserid.org/verify')
{
    if (null === $audience) {
        $audience = $_SERVER['HTTP_HOST'];
    }

    $query = http_build_query(array(
        'assertion' => $assertion,
        'audience' => $audience
    ));

    $opts = array(
        'http' => array(
            'content' => $query,
            'header' => 'Content-Type: application/x-www-form-urlencoded',
            'method' => 'POST'
        )
    );

    $ctx = stream_context_create($opts);
    $contents = file_get_contents($verifyService, false, $ctx);

    if (!$contents) {
        throw new \UnexpectedValueException(sprintf(
            "Verify Service %s did not return a response", $verifyService
        ));
    }

    $data = json_decode($contents);

    if (isset($data->status) and $data->status == 'failure') {
        throw new \UnexpectedValueException(sprintf(
            'The Verify Service returned an error: %s', $data->reason
        ));
    }

    return $data;
}