初学者问个简单的逻辑有关问题，求解答

function dwz($id,$str=""){

	$a=array("0","1","2","3","4","5","6","7","8","9","a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z","A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z");

	$zs=(int)($id/sizeof($a));

	$xs=$id%sizeof($a);

	if($zs>=sizeof($a)){

		$str=$a[$xs].$str;

		dwz($zs,$str);

	}

	else{

		if($str==""){

			return $a[$zs].$a[$xs];

		}

		else{

			echo   $a[$zs].$str;//这里只能输出

			return $a[$zs].$str;//返回没值，不知道什么原因

		}

	}

}

for($i=999990;$i<=1000000;$i++){

	echo dwz($i);

	echo "
";

}

    if($zs>=sizeof($a)){

         $str=$a[$xs].$str;

         dwz($zs,$str); //这里加入return： return dwz($zs, $str);    

}