Two PHP arrays, unset positions that are only present in one array

Question

I have two arrays (in PHP):

ArrayA
(
    [0] => 9
    [1] => 1
    [2] => 2
    [3] => 7
)

ArrayB
(
    [0] => 1
    [1] => 1
    [3] => 8
)

I want to make two new arrays, where I have only the elements declared in both of the arrays, like the following:

ArrayA
(
    [0] => 9
    [1] => 1
    [3] => 7
)

ArrayB
(
    [0] => 1
    [1] => 1
    [3] => 8
)

In this example ArrayA[2] doesn't exist, so ArrayB[2] has been unset.

I wrote this for loop:

for ($i = 0, $i = 99999, $i++){
    if (isset($ArrayA[$i]) AND isset($ArrayB[$i]) == FALSE)
    {
        unset($ArrayA[$i],$ArrayB[$i]);
    } 
}

But it's not great because it tries every index between 0 and a very big number (99999 in this case). How can I improve my code?

Answer 1

The function you're looking for is array_intersect_key:

array_intersect_key() returns an array containing all the entries of array1 which have keys that are present in all the arguments.

Since you want both arrays, you'll have to run it twice, with the parameters in opposite orders, as it only keeps keys from the first array. An example:

$arrayA_filtered = array_intersect_key($arrayA, $arrayB);
$arrayB_filtered = array_intersect_key($arrayB, $arrayA);

Also, although a for loop wasn't ideal in this case, in other cases where you find yourself needing to loop through sparse array (one where not every number is set), you can use a foreach loop:

foreach($array as $key => $value) {
    //Do stuff
}