Is it possible for an array to know its parent? - php

Question

Consider this array:
$super = [
"first" => ["first1","first2"],
"second" => ["second1","second2"]
];
Now let's grab a reference to the subarray:
$sub = $super["second"];
How can I reference the super array from the sub array?
I'm looking for a way to do something like this:
var_dump($sub[../"first"]);
array (size=2)
0 => string 'first1' (length=6)
1 => string 'first2' (length=6)

Answer 1

NO, PHP Arrays are implemented as ordered hashmaps. Which means that every key in the array gets hashed and maps directly to a value. That value could be another array, sure, but the map does not go in reverse. So a value cannot map back to a key in PHP or in the implementation of any typical hashmap. The hash only goes one way.
So in $arr = ["foo" => ["bar"]] the key "foo" in the array $arr will map to the value ["bar"], which happens to be another array. But there is no way to go in reverse order (i.e. map the value ["bar"] back to the key "foo" in $arr).
If you want this kind of co-recursive relationship, like a tree or graph, it's actually easier to accomplish with objects, because objects don't have the same by copy-by-value semantics that arrays do.
$obj = new stdClass;
$obj->first = new stdClass;
$obj->second = new stdClass;
$obj->first->parent = $obj;
$obj->second->parent = $obj;
$obj->first->value = ["first1", "first2"];
$obj->second->value= ["second1","second2"];
$first = $obj->first; // now I can ask it for its parent
$second = $first->parent->second->value;
var_dump($second);
You get
array(2) {
[0]=>
string(7) "second1"
[1]=>
string(7) "second2"
}
Now it doesn't actually matter if you change the value from $first or $obj. The object remains intact, because PHP objects aren't stored directly in the variable. They are stored in a container that's abstracted away from userland and the variable merely holds a reference to this container. So $first and $obj still both point to the same object.
So continuing from the example above, if we tried something like this ...
$second = $first->parent->second;
$first->parent->second->value = ["I changed you!", "I changed you too!"];
// This magically gets the right value
var_dump($second->value, $obj->second->value);
array(2) {
[0]=>
string(14) "I changed you!"
[1]=>
string(18) "I changed you too!"
}
array(2) {
[0]=>
string(14) "I changed you!"
[1]=>
string(18) "I changed you too!"
}

Answer 2

You can only know what's the parent index of an index by creating custom function like this :
function get_parent($key, $arr){
$index = 0;
foreach($arr as $keyCur => $val){
if($keyCur === $key){
$ret = [];
$ret['indexOfKey'] = $index;
$ret['valueOfKey'] = $val;
return $ret;
}
$index++;
}
return false;
}