header('Content-Type: text/html; charset=UTF-8');
<a href="?w=<?=$result['slug']?>">Visible<br></a>
) (Также пытался убирать ?w= в значение "href")<!DOCTYPE html>
<html lang="ru">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Форма регистрации</title>
<link href="https://cdn.jsdelivr.net/npm/bootstrap@5.1.3/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-1BmE4kWBq78iYhFldvKuhfTAU6auU8tT94WrHftjDbrCEXSU1oBoqyl2QvZ6jIW3" crossorigin="anonymous">
<link rel="stylesheet" href="style.css">
<link rel="preconnect" href="https://fonts.googleapis.com">
<link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
<link href="https://fonts.googleapis.com/css2?family=Inter:wght@600&display=swap" rel="stylesheet">
</head>
<body>
<div class="background"></div>
<input class="email" type="text" placeholder="E-mail"/>
<input class="pass" type="text" placeholder="Пароль"/>
<p class="geniussite">genius.site</p>
<div id="square"></div>
<button id="vhod">ВХОД</button>
</body>
</html>