Warning: require_once(header.php) [function.require-once]: failed to open stream: No such file or directory in /public_html/index.php on line 3

Fatal error: require_once() [function.require]: Failed opening required 'header.php' (include_path='.:/usr/local/php5.2/pear:/usr/share/pear') in /public_html/index.php on line 3