Most efficient way to get previous letter in the alphabet using 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