String Increment
String increment is the PHP feature that applies the ++ pre- and post-increment operators on non-numeric strings.
These strings last character’s ASCII code is incremented by one, with possible extension of the string when the letters reach z and Z.
This feature, when based on ++ is deprecated, and shall be removed in version 9. It has been replaced by the str_increment() function, which has the same feature, though not implicit in the engine anymore. It is also extended with It has been replaced by the str_decrement().
A deprecation message is active since PHP 8.3.
<?php
$s = 'y';
echo ++$s; // z
echo ++$s; // aa
echo ++$s; // ab
$s = 'Y';
echo ++$s; // Z
echo ++$s; // AA
echo ++$s; // AA
?>