How variable reference works in PHP: Deep or shallow copy

referenceTest.php

<?php

//example of shallow copy
$arr = array(1, 2);
$arr1 = &$arr;

$arr1[0]++;
echo "\nCopied by reference\n";
echo $arr[0], "\n";
echo $arr1[0], "\n";

unset($arr, $arr1);

//example of deep copy
$arr = array(1, 2);
$arr1 = $arr;

$arr1[0]++;
echo "\nCopied by Value\n";
echo $arr[0], "\n";
echo $arr1[0], "\n";