PHP pass by value and mutation

Question

In the PHP, we have a copy-on-write behavior.

1. Why the following code has an unexpected result? As you can see passByValue function mutate the $value array. Since it's a pass by value, I expect that it must not change the original array passed, Actually is pushes new element in the original array too!
2. What are the differences between $x = &$a['x']; and $x =& $a['x'];

<?php

function passByValue($value){
    $value['x'][] = 2;
}

$a =['x'=>[[1]]];
$x = &$a['x']; // Comment this line, the array is not mutated

passByValue($a);
var_dump($a);
?>

array(1) {
  ["x"]=>
  &array(2) {
    [0]=>
    array(1) {
      [0]=>
      int(1)
    }
    [1]=>
    int(2)
  }
}