bbpm_conversation_form_additional_fields.php

<?php
add_action('bbpm_conversation_form_additional_fields', function() {

	/**
	  * Follows is just an example on how to implement a simple captcha,
	  * You can set the answer in a cookie or database option and fetch
	  * it with a unique key, or store it somewhere else, use honeypot and
	  * spam hidden traps, reCaptcha, or simply a captcha image..
	  */

	if( ! se_bbpm_add_spam_check() )
		return; // not enabled for this user

	$rand1 = rand(5,20); // generate random number
	$rand2 = rand(1,9); // generate random number

	$operations = array(
		'+' => 'plus',
		'-' => 'minus'
		// add more as you wish, be careful with floats while *|/
	);

	$operationRand = array_rand($operations); // get a random operation from operations list

	?>

		<p>
			<label>
				<strong style="display: inline;">Spam test:</strong> What's <?php echo $rand1 . ' ' . $operations[$operationRand] . ' ' . $rand2; ?>? <br/>
				<input type="text" name="capCheck[value]" />
				<input type="hidden" name="capCheck[operation]" value="<?php echo $operations[$operationRand]; ?>" />
				<input type="hidden" name="capCheck[a]" value="<?php echo $rand1; ?>" />
				<input type="hidden" name="capCheck[b]" value="<?php echo $rand2; ?>" />
			</label>
		</p>

	<?php

});