<?php
$verification = false;
$equiv = (($_POST['captcha'] + 18 )/ 4) - $_POST['random'];
if ($equiv == 0)
{$verification = true;}
else {echo "There was an error with your submission. Please retry.";}

if ($verification == true){
// Verification passed - now what do you want to do?
echo "Congratulations, you entered the correct number!";
}
?>