It depends on what you want to do when you click Z, but you have a few options:

$nextChar = chr(ord($currChar) + 1); // "a" -> "b", "z" -> "{"

You can also use PHP’s range() function:

$chars = range('a', 'z');  // ['a', 'b', 'c', 'd', ...]

In my opinion, the most effective way is to only increase the string variable.

$str = 'a';
echo ++$str; // prints 'b'

$str = 'z';
echo ++$str; // prints 'aa'

'a', incrementing 'z' will give 'aa' code> You can simply check the length of the resulting string and if it >1 resets it.

$ch = 'a';
$next_ch = ++$ch; 
if (strlen($next_ch) > 1) { // if you go beyond z or Z reset to a or A
 $next_ch = $next_ch[0];
}