$array = array(
"a1" => array(
"b1" => array(
"d" => 1,
"f" => 2,
),
"b2" => array(
"d" => 3,
"f" => 4,
),
),
"a2" => array(
"b1" => array(
"d" => 1,
"f" => 2,
),
"b2" => array(
"d" => 3,
"f" => 4,
),
),
);
print_r ($array);
$new = [
'a1' => [
'b1' => $array['a1']['b1']
]
];
Если не устраивает, попробуйте подробнее описать задачу.