$source = [10, 20, 30, 40, 50, 60, 70];
$target = [1, 2, 3];
// Add elements from index 2 to 4 (30, 40, 50)
$range = array_slice($source, 2, 3);
$target = [...$target, ...$range];
print_r($target);
/*
run:
Array
(
[0] => 1
[1] => 2
[2] => 3
[3] => 30
[4] => 40
[5] => 50
)
*/