replace a string in wordpress, with acf field

function start_modify_html() {
   ob_start();
}

function end_modify_html() {
  
  if(!is_user_logged_in()) {
    $html = ob_get_clean();
      $html = str_replace( '[paisname]', get_field('paisname', 'option'), $html );
      echo $html;
  }
}

add_action( 'wp_head', 'start_modify_html' );
add_action( 'wp_footer', 'end_modify_html' );