Convert every two values from a flat array into key-value pairs of a flat associative array

I have data in an index array where even index values are keys and odd are values, I am trying to make them key value in new array, see my code below.

Array
(
 [0] => firstName
 [1] => bob
 [2] => lastName
 [3] => alex
)

Code

$k = array();
$v = array();

foreach ($a as $key => $value) {
    if ($key % 2 == 0) {
        $k[] = $value;
    } else {
        $v[] = $value;
    }
}

Here's a solution using array_intersect_key to select the odd-numbered values and then the even-numbered values (using range to generate the list of key values), and then using array_combine to generate the output from those sets of values:

$a = [ 'firstname' , 'bob' , 'lastname' , 'alex' ];

$new = array_combine(
    array_intersect_key($a, array_flip(range(0, count($a) - 1, 2))),
    array_intersect_key($a, array_flip(range(1, count($a) - 1, 2)))
    );

print_r($new);

Output:

Array (
  [firstname] => bob
  [lastname] => alex 
)

Demo on 3v4l.org