$arr["hoge"][0] = "a";
$arr["hoge"][1] = "b";
$arr["fuga"][0] = "1";
$arr["fuga"][1] = "2";
$arr["fuga"][2] = "3";
$arr["geha"][0] = "い";
$arr["geha"][1] = "ろ";
$arr["geha"][2] = "は";
$arr["geha"][3] = "に";
だった場合とか考慮しなくていいの？