<div class="signUp">
<h1>Регистрация</h1>
<form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="POST">
<input type="text" name="login" placeholder="Логин"><br>
<input type="text" name="email" placeholder="E-mail"><br>
<input type="password" name="password" placeholder="Пароль"><br>
<input type="password" name="repeat_password" placeholder="Повторите пароль"><br>
<input type="submit" name="signUp" value="Регистрация">
</form>
</div>
<a href="" id="signInButton">Войти</a>
<a href="" id="signUpButton">Регистрация</a>
<script>
var signUpBut = getElementById('signUpButton');
signUpBut.onclick = function() {
getElementsByClassName('signUp')[0].style.display = state;
}
</script>