WordPress filter to modify final html output

Question

WordPress has great filter support for getting at all sorts of specific bits of content and modifying it before output. Like the_content filter, which lets you access the markup for a post before it's output to the screen.

I'm trying to find a catch-all filter that gives me one last crack at modifying the final markup in its entirety before output.

I've browsed the list of filters a number of times, but nothing jumps out at me: https://codex.wordpress.org/Plugin_API/Filter_Reference

Anyone know of one?

Answer 1

WordPress doesn't have a "final output" filter, but you can hack together one. The below example resides within a "Must Use" plugin I've created for a project.

Note: I haven't tested with any plugins that might make use of the "shutdown" action.

The plugin works by iterating through all the open buffer levels, closing them and capturing their output. It then fires off the "final_output" filter, echoing the filtered content.

Sadly, WordPress performs almost the exact same process (closing the open buffers), but doesn't actually capture the buffer for filtering (just flushes it), so additional "shutdown" actions won't have access to it. Because of this, the below action is prioritized above WordPress's.

wp-content/mu-plugins/buffer.php

<?php

/**
 * Output Buffering
 *
 * Buffers the entire WP process, capturing the final output for manipulation.
 */

ob_start();

add_action('shutdown', function() {
    $final = '';

    // We'll need to get the number of ob levels we're in, so that we can iterate over each, collecting
    // that buffer's output into the final output.
    $levels = ob_get_level();

    for ($i = 0; $i < $levels; $i++) {
        $final .= ob_get_clean();
    }

    // Apply any filters to the final output
    echo apply_filters('final_output', $final);
}, 0);

An example of hooking into the final_output filter:

<?php

add_filter('final_output', function($output) {
    return str_replace('foo', 'bar', $output);
});

Edit:

This code uses anonymous functions, which are only supported in PHP 5.3 or newer. If you're running a website using PHP 5.2 or older, you're doing yourself a disservice. PHP 5.2 was released in 2006, and even though Wordpress (edit: in WP version < 5.2) STILL supports it, you should not use it.