If an associative array is used as the second parameter of array_fill_keys, then the associative array will be appended in all the values of the first array.
e.g.
<?php
$array1 = array(
"a" => "first",
"b" => "second",
"c" => "something",
"red"
);
$array2 = array(
"a" => "first",
"b" => "something",
"letsc"
);
print_r(array_fill_keys($array1, $array2));
?>
The output will be
Array(
[first] => Array(
[a] => first,
[b] => something,
[0] => letsc
),
[second] => Array(
[a] => first,
[b] => something,
[0] => letsc
),
[something] => Array(
[a] => first,
[b] => something,
[0] => letsc
),
[red] => Array(
[a] => first,
[b] => something,
[0] => letsc
)
)