Cookie间歇性地丢失PHP中的值

An error has cropped up on our site (PHP/Linux/Apache) where the $_COOKIE global variable loses all its values on certain pages. This is usually after either following a link with GET parameters or submitting a POST form. I'm really puzzled as to why this has suddenly started occuring. To illustrate, I have dumped out the values of the $_COOKIE variable on each page as I navigate through the site (some values replaced by xxx).

Home page (all fine):

Array
(
    [nav-hosting] => 1
    [nav-server_management] => 1
    [locale] => en-US
    [psaContext] => tools
    [mycart] => xxx
    [myid] => xxx
    [__utma] => xxx
    [__utmb] => xxx
    [__utmc] => xxx
    [__utmz] => xxx
)

Results page after submitting site search form (all fine):

Array
(
    [nav-hosting] => 1
    [nav-server_management] => 1
    [locale] => en-US
    [psaContext] => tools
    [mycart] => xxx
    [myid] => xxx
    [__utma] => xxx
    [__utmb] => xxx
    [__utmc] => xxx
    [__utmz] => xxx
)

Click through to a page from the search results (problem occurs):

Array
(
    [1] => 
)

So, the $_COOKIE inexplicably has an index of 1 with a null value. Can anyone suggest what might be causing this?

A further click from that last page with the problem will resume all the original values again. Baffling!

As you did not provide any code, I´ll guess: Are you on another subdomain, or moved out of the path where your cookie is valid?