PHP Memory Leak

Question

Looking for a little help as Apache is complaining about this php code causing memory leaks. Does anyone see anything out of the ordinary happening here? Specifically at $output .= ob_get_clean();

See code below;

function start_el( &$output, $comment, $depth = 0, $args = array(), $id = 0 ) {
    $depth++;
    $GLOBALS['comment_depth'] = $depth;
    $GLOBALS['comment'] = $comment;

    if ( !empty( $args['callback'] ) ) {
        ob_start();
        call_user_func( $args['callback'], $comment, $args, $depth );
        $output .= ob_get_clean();
        return;
    }

    if ( ( 'pingback' == $comment->comment_type || 'trackback' == $comment->comment_type ) && $args['short_ping'] ) {
        ob_start();
        $this->ping( $comment, $depth, $args );
        $output .= ob_get_clean();
    } elseif ( 'html5' === $args['format'] ) {
        ob_start();
        $this->html5_comment( $comment, $depth, $args );
        $output .= ob_get_clean();
    } else {
        ob_start();
        $this->comment( $comment, $depth, $args );
        $output .= ob_get_clean();
    }
}