<form action="login.php">
<input name="login" type="text"><br><br>
<input name="password" type="password"><br>
<button name="submit" id="enter">Войти</button>
</form>
<?php
if(empty($login) and empty($password)){
}
else{
echo "Привет, <strong>".$login."</strong> | <a href='exit.php'>Выход</a>";
}
?>