Wrap functions hooked to a WordPress action hook with HTML comments

wrap-hook-actions-with-comments.php

<?php
/**
 * Wraps output from functions attached to the wp_footer hook with HTML comments
 * Useful for finding stray bits of output. Save as an MU plugin and change the hook to whatever you want.
 */
 
add_action( 'wp_footer', function() {
	echo '<!-- Start wp_footer -->' . "\n";
	global $wp_filter;

	if ( isset( $wp_filter['wp_footer'] ) && is_a( $wp_filter['wp_footer'], 'WP_Hook' ) ) {
		foreach ( $wp_filter['wp_footer']->callbacks as $priority => $callbacks ) {
			foreach ( $callbacks as $key => $callback ) {
				$original_callback = $callback['function'];

				// Wrap the original callback to output HTML comments before and after execution
				$wp_filter['wp_footer']->callbacks[ $priority ][ $key ]['function'] = function() use ( $original_callback ) {
					if ( is_array( $original_callback ) ) {
						$function_name = is_object( $original_callback[0] )
							? get_class( $original_callback[0] ) . '->' . $original_callback[1]
							: $original_callback[0] . '::' . $original_callback[1];
					} elseif ( is_string( $original_callback ) ) {
						$function_name = $original_callback;
					} else {
						$function_name = 'Closure or anonymous function';
					}

					// Output HTML comment before execution
					echo "<!-- Before executing: $function_name -->\n";

					// Call the original callback
					call_user_func_array( $original_callback, func_get_args() );

					// Output HTML comment after execution
					echo "<!-- After executing: $function_name -->\n";
				};
			}
		}
	}
  echo '<!-- End wp_footer -->' . "\n";
}, -9999 );