(PECL apc >= 3.1.4)
apc_exists — Checks if APC key exists
   Returns TRUE if the key exists, otherwise FALSE Or if an
   array was passed to keys, then
   an array is returned that contains all existing keys, or an empty
   array if none exist.
  
Example #1 apc_exists() example
<?php
$fruit  = 'apple';
$veggie = 'carrot';
apc_store('foo', $fruit);
apc_store('bar', $veggie);
if (apc_exists('foo')) {
    echo "Foo exists: ";
    echo apc_fetch('foo');
} else {
    echo "Foo does not exist";
}
echo PHP_EOL;
if (apc_exists('baz')) {
    echo "Baz exists.";
} else {
    echo "Baz does not exist";
}
echo PHP_EOL;
$ret = apc_exists(array('foo', 'donotexist', 'bar'));
var_dump($ret);
?>
The above example will output something similar to:
Foo exists: apple
Baz does not exist
array(2) {
  ["foo"]=>
  bool(true)
  ["bar"]=>
  bool(true)
}