Cant use ob_start param

<?php
    if((!file_exists($page_part))&&($cache=='y')):
        ob_start();
?>
<!DOCTYPE html>
<html lang="it">
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<?php
        $html=sanitize_all(ob_get_contents());
        $fp=fopen($page_part,'w');
        fwrite($fp,$html);
        fclose($fp);
        ob_end_clean();
    endif;
    require_once($page_part);