Polyfill for PHP function getallheaders() (alias apache_request_headers())

polyfill_getallheaders.php

<?php

if (!\function_exists('getallheaders')) {
    /**
     * Returns HTTP headers for the server request.
     * 
     * @return string[] The HTTP headers
     */
    function getallheaders()
    {
        /** @var string[]|null $headers */
        static $headers;
        
        if (!isset($headers)) {
            $headers = [];
            $extraHeaders = [
                'CONTENT_TYPE'    => 'Content-Type',
                'CONTENT_LENGTH'  => 'Content-Length',
                'CONTENT_MD5'     => 'Content-MD5',
                'PHP_AUTH_DIGEST' => 'Authorization',
            ];
            /** @var array $server */
            $server = \filter_input_array(\INPUT_SERVER, \FILTER_SANITIZE_STRING);
            foreach ($server as $key => $value) {
                if (isset($extraHeaders[$key])) {
                    $headers[$extraHeaders[$key]] = $value;
                } elseif (\strncmp($key, 'HTTP_', 5) === 0) {
                    $key = \strtr(\substr($key, 5), ['_' => '-']);
                    $key = \ucwords(\strtolower($key), '-');
                    $headers[$key] = $value;
                }
            }

            if (
                !isset($headers['Authorization']) 
                && (isset($server['AUTH_TYPE']) || isset($server['PHP_AUTH_USER']))
            ) {
                $headers['Authorization'] = isset($server['AUTH_TYPE']) ? $server['AUTH_TYPE'] : 'Basic';
                if (isset($server['PHP_AUTH_USER'])) {
                    $token = $server['PHP_AUTH_USER'] . ':' . (isset($server['PHP_AUTH_PW']) ? $server['PHP_AUTH_PW'] : '');
                    $headers['Authorization'] .= ' ' . \base64_encode($token);
                }
            }
        }
        
        return $headers;
    }
}