Negative Index
On PHP strings, a positive index starts at zero at the beginning of the string; and a negative index starts at the end of the string.
Negative index was added in version 7.1.
On an array, negative indexes are supported, without any special meaning: they are only available when provided. Their behavior changed when using automatic indexing though.
<?php
$string = 'abcd';
echo $string[1]; // b
echo $string[-1]; // c
?>