Replace a list table in the WordPress admin

replace_comments_list_table.php

<?php

// Add action for admin_head-edit-comments.php

/*
 * This action is triggered by admin-header.php when the user visits 
 * edit-comments.php and will execute after $wp_list_table is initially defined.
 */

add_action("admin_head-edit-comments.php", function() {

	// Create a custom class that extends WP_Comments_List_Table
	
	/*
	 * For this example I've defined the class inside the action callback. In a 
	 * real world scenario I would probably put the class in its own seperate 
	 * file and include that file here.
	 *
	 * Because the original $wp_list_table was already defined using an instance 
	 * of WP_Comments_List_Table when the admin_head-edit-comments.php action is 
	 * triggered I don't have to bother about including other files. If you 
	 * would define your custom class somewhere outside the callback you might 
	 * need to include these files:
	 *
	 * - wp-admin/includes/class-wp-list-table.php
	 * - wp-admin/includes/class-wp-comments-list-table.php
	 */

	class MyCommentsListTable extends WP_Comments_List_Table {

		// Replace the single_row() method from the original WP_Comments_List_Table
		
		/*
		 * In this example I'll replace the single_row() method which is called 
		 * for each row in the list table.
		 *
		 * To get a better understanding of all available methods you might want 
		 * to take a look at the comments for the original WP_List_Table and the 
		 * definitions of methods for WP_Comments_List_Table.
		 */

		function single_row($comment) {
			echo '<tr><td colspan="' . $this->get_column_count() . '">' . 
				"My Custom Row for comment with ID {$comment->comment_ID} by {$comment->comment_author}" . 
				'</td></tr>';
		}

	}

	// Add global $wp_list_table to current scope

	global $wp_list_table;

	// Create an instance of your custom list table class

	$myListTable = new MyCommentsListTable;

	// Copy all properties from the global list table to yours

	foreach ($wp_list_table as $key => $value)
		$myListTable->{$key} = $value;

	// Replace the global list table with yours

	$wp_list_table = $myListTable;
});