Full page modification in WordPress

wp_full_page_modification.php

<?php
  add_action('init', 'fpm_start_output');
  add_action('shutdown', 'fpm_end_output');

  function fpm_start_output() {
    ob_start('fpm_mod_buffer');
  }
  
  function fpm_mod_buffer($buffer) {
    // Modification ... regexp, etc 
    return $buffer;
  }
  
  function fpm_end_output() {
    ob_end_flush();
  }