Trying to make a script that showed the letters from A to Z using the following code:

   for ($i = 'A'; $i <= 'Z'; $i++) {
      echo "$i<br>";      
   }

I saw that the script outputted the first letters (A-Z) but then it started to display AA, AB, AC [...] YZ.
Replacing the 'Z' above to 'Y' (or any other letter between B and Y) makes the script show correctly the letters from A to that letter. 
Replacing letters to lowercase and/or single quotation marks to double causes the same error.

This happened both in PHP 4.0.4pl1 under Windows and in PHP 4.0.1pl1 under Linux.