//code by liuyuanjun.com
function LetterAdd($s){
	$Str = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
	$len = strlen($s);
	$i = 1;
	do{
		$a2 = substr($s,$len-$i,1);
		$pos = strpos($Str,$a2)+1;
		$b2 = $pos>25 ? "A" : $Str[$pos];
		$a1 = $len==$i ? ($b2=="A"?"A":"") : substr($s,0,$len-$i);
		$a3 = $i==1 ? "" : substr($s,$len-$i+1);
		$s = $a1.$b2.$a3;
		$i++;
	}while($b2=="A" && $len>$i-1);
	return $s;
}

for($i=A;$i!="AAA";$i=LetterAdd($i)){
	echo $i."\n";
}