Applying ++ to a string essentially treats it like a base-26 number. It goes from 'a,b,c... y,z,aa,ab,ac...' etc. 

Try something like this. Maybe some else has a tidier way?

  for ( $i='A'; $i!='AA'; $i++ )
    print $i. '<br>';

i assume your point is that <= does not operate on letters in the same way; it's a string comparison operator.  i should have seen this, i suppose.

it's too late now, but perhaps php should have been developed more along the lines of perl, with separate operators for string functions... then mathematical operators could treat letters as base 26 numbers, and string operators could do alphabetical comparisons.

this special overloading of ++ is misleading.