Simple honeypot for WP comments

functions.php

<?php
// We will add a fake input field to the WP comment form and make it invisible for humans
// The "firstname" field name is not used by WP (by default) so we can use it
function simple_honey_pot_44587513(){
	ob_start();
	?>
	<style>
	.rpodum {
		top: 0;
		left: 0;
		width: 0;
		height: 0;
		opacity: 0;
		z-index: -1;
		position: absolute;
	}
	</style>
	<input name="firstname" type="text" id="firstname" class="rpodum" autocomplete="off" tabindex="-1" />
	<?php
	echo ob_get_clean();
}
add_action('comment_form', 'simple_honey_pot_44587513');

// Since our fake field is hidden for human eyes, it will not be populated by human
// Robots will fill all fields inside the form - even invisible ones!
function check_simple_honey_pot_44587513($approved) {
	// If our invisible field IS empty, then everything is good
	// If it is NOT empty, mark the comment as "spam"
	return (empty($_POST['firstname'])) ? $approved : 'spam'; 
}
add_filter('pre_comment_approved', 'check_simple_honey_pot_44587513', 9999, 1);