automate query and execute mail()in cronjob

[dflow]

how can i execute mail() according to a certain date (+plus 4 days)

for example:

<?php 
$today= echo 'date("Y-m-j")';
$query_SendFeedbackForm=sprintf("SELECT * FROM contact_form WHERE ReturnDate ='$today'-'4';")
?>
//if $today=ReturnDate+4days ==>send feedback form with(cronjob) :
   <?php  
   if ($today==$row_SendFeedbackForm['ReturnDate']+'4') {?>

$to=$row_SendFeedbackForm['CustomerEmail'];
   mail($to, $subject, $message, $headers); 

<?php } else { ?>
nada
<?php } ?>