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
}