public function __construct()
{

    add_action( 'wp_head', array( $this, 'start') );
    add_action( 'wp_print_footer_scripts', array( $this, 'end') );

} // end constructor

public function start() { ob_start( array( $this, 'callback' ) ); } // end start

public function end() { ob_end_flush(); } // end end

public function callback( $buffer )
{

    // do some stuff

    return $buffer;

} // end callback