Results of Array Spread Operator with explicit numeric keys in arrays

Code
<?php

$a 
= [=> 1=> 2];
$b = [...$a=> 3=> 4];

dump($b);
Result
array:3 [
0 => 1
1 => 4
3 => 3
]
Used Versions
PHP 7.4, Zend 3.0, Symfony 4.3, Laravel 5.8, PHPUnit 8.0, Doctrine 2.10