A proper drop-in replacement for apache_request_headers() when that's not available

apache_request_headers.php

<?php

if(!function_exists('apache_request_headers')) {
///
    function apache_request_headers() {
        // Based on: http://www.iana.org/assignments/message-headers/message-headers.xml#perm-headers
        $arrCasedHeaders = array(
            // HTTP
            'Dasl'             => 'DASL',
            'Dav'              => 'DAV',
            'Etag'             => 'ETag',
            'Mime-Version'     => 'MIME-Version',
            'Slug'             => 'SLUG',
            'Te'               => 'TE',
            'Www-Authenticate' => 'WWW-Authenticate',
            // MIME
            'Content-Md5'      => 'Content-MD5',
            'Content-Id'       => 'Content-ID',
            'Content-Features' => 'Content-features',
        );
        $arrHttpHeaders = array();
 
        foreach($_SERVER as $strKey => $mixValue) {
            if('HTTP_' === substr($strKey, 0, 5)) {
                $strHeaderKey = strtolower(substr($strKey, 5));
                $arrHeaderKey = explode('_', $strHeaderKey);
 
                if(0 < count($arrHeaderKey)) {
                    $arrHeaderKey = array_map('ucfirst', $arrHeaderKey);
                    $strHeaderKey = implode('-', $arrHeaderKey);
                }
                else {
                    $strHeaderKey = ucfirst($strHeaderKey);
                }
 
                if(array_key_exists($strHeaderKey, $arrCasedHeaders)) {
                    $strHeaderKey = $arrCasedHeaders[$strHeaderKey];
                }
 
                $arrHttpHeaders[$strHeaderKey] = $mixValue;
            }
        }
 
        return $arrHttpHeaders;
    }
///
}