昨日は金曜日
今日は土曜日
明日は日曜日

<?php
mb_language("ja");
mb_internal_encoding("UTF-8");
$youbi = array("日","月","火","水","木","金","土"); //曜日を配列に入れる
$today = date("Y/n/d"); //今日
$you = date("w"); //曜日
$back = date("w")-1;
if ($back < 0) {$back = 6;}
$next = date("w")+1;
if ($next > 6) {$next = 0;}
?>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>昨日今日明日の曜日</title>
</head>
<body>
<?php
print "昨日は" . $youbi[$back] . "曜日<br>\n";
print "今日は" . $youbi[$you] . "曜日<br>\n";
print "明日は" . $youbi[$next] . "曜日<br>\n";
?>
</body>
</html>