(PECL apcu >= 5.1.0)
apcu_entry — Atomically fetch or generate a cache entry
   Atomically attempts to find key in the cache, if it cannot be found generator
   is called, passing key as the only argument. The return value of the call is then cached with the optionally specified
   ttl, and returned.
  
Note: When control enters apcu_entry() the lock for the cache is acquired exclusively, it is released when control leaves apcu_entry(): In effect, this turns the body of
generatorinto a critical section, disallowing two processes from executing the same code paths concurrently. In addition, it prohibits the concurrent execution of any other APCu functions, since they will acquire the same lock.
    The only APCu function that can be called safely by generator is apcu_entry().
   
keyIdentity of cache entry
generator
       A callable that accepts key as the only argument and returns the value to cache.
      
ttl
       Time To Live; store var in the cache for
       ttl seconds. After the
       ttl has passed, the stored variable will be
       expunged from the cache (on the next request). If no ttl
       is supplied (or if the ttl is
       0), the value will persist until it is removed from
       the cache manually, or otherwise fails to exist in the cache (clear,
       restart, etc.).
      
Returns the cached value
Example #1 An apcu_entry() example
<?php
$config = apcu_entry("config", function($key) {
 return [
   "fruit" => apcu_entry("config.fruit", function($key){
     return [
       "apples",
       "pears"
     ];
   }), 
   "people" => apcu_entry("config.people", function($key){
     return [
      "bob",
      "joe",
      "niki"
     ];
   })
 ];
});
var_dump($config);
?>
The above example will output:
array(2) {
  ["fruit"]=>
  array(2) {
    [0]=>
    string(6) "apples"
    [1]=>
    string(5) "pears"
  }
  ["people"]=>
  array(3) {
    [0]=>
    string(3) "bob"
    [1]=>
    string(3) "joe"
    [2]=>
    string(4) "niki"
  }
}