Hook that passes/fails a PR based upon whether we updated the changelog

changelog.php

<?php

$secret = '...';
$token = '...';

$payload = $_POST['payload'];
$signature_header = $_SERVER['HTTP_X_HUB_SIGNATURE'];
$signature_calc = 'sha1=' . hash_hmac('sha1', 'payload=' . urlencode($payload), $secret, false);
if (!hash_equals($signature_header, $signature_calc)) {
    exit("Signature did not match");
}

$data = json_decode($payload);
if (!$data) {
    exit("No JSON data");
}

$actions = array('opened', 'reopened', 'synchronize', 'edited');
if (!in_array($data->action, $actions)) {
    exit("Ignored action");
}

$pr = $data->pull_request;
if (want_to_check($pr)) {
    print "Checking PR\n";
    if (want_to_skip($pr)) {
        $result = 'skip';
    } elseif (changelog_changed($pr)) {
        $result = 'pass';
    } else {
        $result = 'fail';
    }
    print "PR $result\n";
    set_changelog_status($pr, $result);
    print "Status set\n";
} else {
    print "Ignoring PR\n";
}

function want_to_check($pr) {
    return $pr->state == 'open' && $pr->base->ref == 'master';
}

function want_to_skip($pr) {
    return preg_match('#\[skip changelog\]#', $pr->body);
}

function changelog_changed($pr) {
    $diff = file_get_contents($pr->diff_url);
    if (preg_match('#diff --git a/CHANGELOG.md#', $diff)) {
        return true;
    }
    return false;
}

function set_changelog_status($pr, $result) {
    $msgs = array(
        'fail' => 'No changelog update found',
        'pass' => 'Changelog update found',
        'skip' => 'Changelog check skipped',
    );
    set_commit_status($pr->statuses_url, $result != 'fail' ? 'success' : 'failure', $msgs[$result], 'changelog');
}

function set_commit_status($url, $state, $desc, $context) {
    global $token;

    $ch = curl_init();
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch, CURLOPT_HTTPAUTH, CURLAUTH_BASIC);
    curl_setopt($ch, CURLOPT_USERAGENT, "mySociety Changelog bot");

    $data = array(
        'state' => $state,
        'description' => $desc,
        'context' => $context,
    );

    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_POST, true);
    curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($data));
    curl_setopt($ch, CURLOPT_HTTPHEADER, array(
        'Content-Type: application/json',
        "Authorization: token $token",
    ));
    curl_exec($ch);
    curl_close($ch);
}