<div>Server error itu sangat luas, bisa kesalaha dari mana saja.<br>Salah satu cara debugnya dengan mengaktifkan error PHP, agar ketahuan masalahnya di mana jika dari kodenya</div>

<pre>&lt;?php

require 'functions.php';

if(isset($_POST["login"]) ) {


	$username = $_POST["username"];
	$password = $_POST["password"];

	$result = mysqli_query($conn,"SELECT * FROM user WHERE username = '$username'");


	//cek username
	if(mysqli_num_rows($result) === 1) {

		//cek password 
		$row = mysqli_fetch_assoc($result);
		if(password_verify($password, $row["password"])) {
			header("Location : index.php");
			exit;
		}

	}

	$error = true;

}

?&gt;

&lt;!DOCTYPE html&gt; &lt;html&gt; &lt;head&gt; &lt;title&gt;Halaman Login&lt;/title&gt; &lt;/head&gt; &lt;body&gt;

<h1 >Halaman Login</h1>

<?php if(isset($error)) : ?>
	<p style="color: red; font-style: italic;">Username atau Password yang dimasukkan salah ! </p>
<?php endif; ?>

<form action="" method="post">
	
	<ul>
		<li>
			<label for="username">Username</label>
			<input type="text" name="username" id="username">
		</li>
		<li>
			<label for="password">Password</label>
			<input type="password" name="password" id="password">
		</li>
		<li>
			<button type="submit" name="login">Login</button>
		</li>
	</ul>

</form> 

&lt;/body&gt; &lt;/html&gt;</pre>