Description:
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Hi there.

Our policy prevents us from using CVS level code *grumble* however, I believe I have come up with what might be an acceptable mitigation method for cache slam defense (ie: bug16814)

I'm aware of some of the pro's and cons with this method, but it seems to provide expected results.

cache slam defense - have issues writing and overwriting.
non cache slam defense - huge issue that can lead to explosion of apache
alternate defense - can still have explosion of apache, but hopefully not as extreme due to one write point...

If it's not convenient to do in module, or possible, well that's ok, but I thought I'd suggest it anyway.

Reproduce code:
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function cache_store($entry, $value, $ttl = null) {
    $GLOBALS['please_cache_me'][$entry] = array(
        'value' => $value,
        'ttl'   => $ttl
    );
}

function cache_fetch($entry) {
    if (array_key_exists($entry, $GLOBALS['please_cache_me'])) return $GLOBALS['please_cache_me'][$entry]['value'];
    return apc_fetch($entry);
}

function cache_shutdown() {
    foreach ($GLOBALS['please_cache_me'] as $entry => $data) {
        apc_store($entry, $data['value'], $data['ttl']);
    }
}

register_shutdown_function('cache_shutdown');

cache_store('entry', 'value');
cache_store('entry', 'doh');
echo cache_fetch('entry');

Expected result:
----------------
doh

Actual result:
--------------
doh