How to build this php array?

Question

$items  = "word1 word2 word3 word4";
$items = explode(" ", $items);

$items is my array, how can I have it turn into this

Like how can I make word1 be a key and a value?

"word1" => "word1"

Answer 1

$newArray = array_combine($items, $items);

Answer 2

foreach($items as $item) {
$newArr[$item] = $item;
}

Answer 3

$items  = "word1 word2 word3 word4";
$temp = explode(" ", $items);
$result = array();
foreach ($temp as $value) {
  $result[$value] = $value; 
}