$cookieName = "city";
$cookieValue = "서울";
// 쿠키를 삭제함. unset($_COOKIE["cookie"])와 같음.
setcookie($cookieName, $cookieValue, time()-60, "/");
<html lang="ko">
<head>
<meta charset="UTF-8">
<title>PHP Cookie And Session</title>
</head>
<body>
echo "{$cookieName}라는 이름의 쿠키가 삭제되었습니다.";
</body>
</html>