Citation :
 
  When deleting a cookie you should assure that the expiration date is in the past, to trigger the removal mechanism in your browser. Examples follow how to delete cookies sent in previous example:
   Example 2. setcookie() delete example
 <?php
 // set the expiration date to one hour ago
 setcookie ("TestCookie", "", time() - 3600);
 setcookie ("TestCookie", "", time() - 3600, "/~rasmus/", ".example.com", 1);
 ?> 
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