array_filter preserve keys

<?php

$a = ["foo", "bar"];

$b = array_filter($a, function($item) {
    return $item[0] == "f";
});

$c = array_filter($a, function($item) {
    return $item[0] == "b";
});

echo json_encode($a, JSON_PRETTY_PRINT);
echo "\n\n";
echo json_encode($b, JSON_PRETTY_PRINT);
echo "\n\n";
echo json_encode($c, JSON_PRETTY_PRINT);