How to get the intersection of two arrays by keys?

I want to save certain values from $_POST into a file. But I only want values where the key is in the array $lang.

As an example:

$_POST = [1 => "a", 2 => "b", 3 => "c"];
$lang = [2, 3];

With this input I would only want the values from $_POST where the key is in the $lang array.

Expected output would be:

[2 => "b", 3 => "c"]

Right now I'm trying to archive this using ArrayIterator and MultipleIterator, but this loops through both arrays:

$post = new ArrayIterator($_POST);
$lang_array = new ArrayIterator($lang); 
$it = new MultipleIterator;
$it->attachIterator($post);
$it->attachIterator($lang_array);
$fh = fopen('name.php', 'w');
foreach($it as $e) {
    fwrite($fh , $e[1] .'-' . $e[0] );
    fwrite($fh ,"\n" );   
}

So I'm a bit stuck how to solve this problem?

Try this :

// Combining both arrays into one.
$combined_array = array_merge($_POST, $lang);
$fh = fopen('name.php', 'w');
foreach($combined_array as $key => $value){
    fwrite($fh , $key .'-' . $value );
    fwrite($fh ,"\n" );
}