PHP cookie writes on incorrect domain

Question

I have a cookie that I use on my app. It looks like this:

+-------+-------+-----------------------+-------+----------+
| Name  | Value | Domain                | Path  | Expires  |
+-------+-------+-----------------------+-------+----------+
| foo   | bar   | my.domain.tld         | /     | Session  |
+-------+-------+-----------------------+-------+----------+

In a section of my script, based on some condition, I'm trying to change the value of a cookie. I'm using this code:

// overwrite cookie
if($condition){
  setcookie("foo", "cat", 0, "/", "my.domain.tld");
}

Afterward, my cookie data looks like this:

+-------+-------+-----------------------+-------+----------+
| Name  | Value | Domain                | Path  | Expires  |
+-------+-------+-----------------------+-------+----------+
| foo   | bar   | my.domain.tld         | /     | Session  |
| foo   | cat   | .my.domain.tld        | /     | Session  |
+-------+-------+-----------------------+-------+----------+

How come a . is be prepended to the domain? I want to overwrite the existing cookie.

Answer 1

http://www.php.net/manual/en/function.setcookie.php#93641

The answer is discussed in a post on the php manual.

Cookie data is set by the browsing agent, and so is handled differently depending on the process the browser uses.