I found a strange issue when storing then retrieving an associative array via APCu. I have a feeling it has to do with the internal serialize/deserialize routines, so I don’t know if this is a known or expected behavior. Key-matching on the fetched array is not working:

• PHP 8.2.25 on an x86_64 running RH Enterprise Linux 9.5
• APCu version 5.1.23 with PHP serializer

// build superset array
$arOne = [
    "cat"  => ["meow", "cat is hungry"],
    "dog"  => ["woof", "dog is concerned"],
    "bird" => ["chirp", "bird is happy"],
];

// specify subset keys to look for
$arTwo = array_flip( ["dog", "bird"] );

// get a subset array,
// by finding the keys that match in the superset array
$arDirect = array_intersect_key( $arOne, $arTwo );

// store our superset array into APCU
apcu_store("animals",$arOne,60000);

// get a copy of our superset array, from APCU
$arOneFetch = apcu_fetch("animals");

// get a second subset array,
// by finding the keys that match in the copy of our superset array
$arFetch = array_intersect_key( $arOneFetch, $arTwo );

Results:

count($arOne)      is: 3 / JSON Text: {"cat":["meow","cat is hungry"],"dog":["woof","dog is concerned"],"bird":["chirp","bird is happy"]}

count($arOneFetch) is: 3 / JSON Text: {"cat":["meow","cat is hungry"],"dog":["woof","dog is concerned"],"bird":["chirp","bird is happy"]}

count($arDirect)   is: 2 / JSON Text: {"dog":["woof","dog is concerned"],"bird":["chirp","bird is happy"]}

count($arFetch)    is: 0 / JSON Text: []  // <-- I expected this result to be the same as $arDirect

var_dump($arOne):

array (size=3)
  'cat' => 
    array (size=2)
      0 => string 'meow' (length=4)
      1 => string 'cat is hungry' (length=13)
  'dog' => 
    array (size=2)
      0 => string 'woof' (length=4)
      1 => string 'dog is concerned' (length=16)
  'bird' => 
    array (size=2)
      0 => string 'chirp' (length=5)
      1 => string 'bird is happy' (length=13)

var_dump($arOneFetch):

array (size=3)
  'cat' => 
    array (size=2)
      0 => string 'meow' (length=4)
      1 => string 'cat is hungry' (length=13)
  'dog' => 
    array (size=2)
      0 => string 'woof' (length=4)
      1 => string 'dog is concerned' (length=16)
  'bird' => 
    array (size=2)
      0 => string 'chirp' (length=5)
      1 => string 'bird is happy' (length=13)

My workaround was to store the array as JSON encoded, then decode after fetch…but I would rather understand why this is happening.