Most efficient way to get previous letter in the alphabet using PHP - php

Question

Its working nicely:
$str = 'a';
echo ++$str; // prints 'b'
$str = 'z';
echo ++$str; // prints 'aa'
Its very useful to get next column name in an excel file.
But if I use similar code using -- operator to get the previous letter then its not working:
$str = 'b';
echo --$str; // prints 'b' but I need 'a'
$str = 'aa';
echo --$str; // prints 'aa' but I need 'z'
What can be the solution to get the previous letter similarly?
And what can be the reason as its not working?

Answer 1

$str='z';
echo chr(ord($str)-1); //y
Note: This isn't circular for a-z. Need to add rules for that
Fiddle
Edit
This edit covers for your special requirement from excel example. Although its a little longer piece of code.
//Step 1: Build your range; We cant just go about every character in every language.
$x='a';
while($x!='zz') // of course you can take that to zzz or beyond etc
{
$values[]=$x++; // A simple range() call will not work for multiple characters
}
$values[]=$x; // Now this array contains range `a - zz`
//Step 2: Provide reference
$str='ab';
//Step 3: Move next or back
echo $values[array_search(strtolower($str),$values)-1]; // Previous = aa
echo $values[array_search(strtolower($str),$values)+1]; // Next = ac
Fiddle

Answer 2

Using array:
$cla=array('A', 'B', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O',
'P', 'R', 'S', 'Š', 'Z', 'Ž', 'T', 'U', 'V', 'Õ', 'Ä', 'Ö', 'Ü');
$direction = -1;
$element = 'Ü';
$element2 = $cla[((array_search($element,$cla)+count($cla)+$direction)%count($cla))];

Answer 3

I could solve in this way.
How is it?
The cons is that it only can handle uppercase right now.
Some more work can also fix that.
<?php
function get_previous_letter($string){
$last = substr($string, -1);
$part=substr($string, 0, -1);
if(strtoupper($last)=='A'){
$l = substr($part, -1);
if($l=='A'){
return substr($part, 0, -1)."Z";
}
return $part.chr(ord($l)-1);
}else{
return $part.chr(ord($last)-1);
}
}
echo get_previous_letter("AAAAAA");
?>
CODEPAD