How can I post form checkbox data using simple php?

Hi I am new to PHP and decided to try and create a form for the website I'm creating. I managed to do this to go to my mail address apart from sending checkbox data - if more than one checkbox is checked only the last checkbox checked is displayed in the posted email. Can anyone let me know how I can do this? Here is the form code in http://ift.tt/1ENtjuG:

..... Services interested in? Website Logo Stationery Advert Other ... Security Task: 6 + 7 = -->

Here is the post php script (http://ift.tt/1ENtjuK):

$to = "julian@i4dsign.co.uk";
$subject = "New Contact Form Submission";
$message = "....\n\nService: $service[$i]\n\n....";

if ($security=="13"){
    mail($to,$subject,$message);    
    header("Location:contact.php?s=1");
}
else{
    header("Location:contact.php?s=2");
}

Thank you in advance. I hope someone can help.