Aerys\Root::checkPreconditions PHP Method

    private function checkPreconditions(Request $request, int $mtime, string $etag)
    {
        $ifMatch = $request->getHeader("If-Match");
        if ($ifMatch && \stripos($ifMatch, $etag) === false) {
            return self::PRECOND_FAILED;
        }
        $ifNoneMatch = $request->getHeader("If-None-Match");
        if ($ifNoneMatch && \stripos($ifNoneMatch, $etag) !== false) {
            return self::PRECOND_NOT_MODIFIED;
        }
        $ifModifiedSince = $request->getHeader("If-Modified-Since");
        $ifModifiedSince = $ifModifiedSince ? @\strtotime($ifModifiedSince) : 0;
        if ($ifModifiedSince && $mtime > $ifModifiedSince) {
            return self::PRECOND_NOT_MODIFIED;
        }
        $ifUnmodifiedSince = $request->getHeader("If-Unmodified-Since");
        $ifUnmodifiedSince = $ifUnmodifiedSince ? @\strtotime($ifUnmodifiedSince) : 0;
        if ($ifUnmodifiedSince && $mtime > $ifUnmodifiedSince) {
            return self::PRECOND_FAILED;
        }
        $ifRange = $request->getHeader("If-Range");
        if ($ifRange === null || !$request->getHeader("Range")) {
            return self::PRECOND_OK;
        }
        /**
         * This is a really stupid feature of HTTP but ...
         * If-Range headers may be either an HTTP timestamp or an Etag:
         *
         *     If-Range = "If-Range" ":" ( entity-tag | HTTP-date )
         *
         * @link https://tools.ietf.org/html/rfc7233#section-3.2
         */
        if ($httpDate = @\strtotime($ifRange)) {
            return $httpDate > $mtime ? self::PRECOND_IF_RANGE_OK : self::PRECOND_IF_RANGE_FAILED;
        }
        // If the If-Range header was not an HTTP date we assume it's an Etag
        return $etag === $ifRange ? self::PRECOND_IF_RANGE_OK : self::PRECOND_IF_RANGE_FAILED;
    }