In my opinion, you should use a database to store all your credentials (like username, password, etc..)
If you don't know how to do it, you should know that if you want to run your php code, you need a php server and somewhere to put your db.
Here is how to set up a php server with Apache
https://www.ultraedit.com/support/tutorials-power-tips/uestudio/local-php-mysql-dev-environment.html
Here is how to set up a db with PhpMyAdmin
https://www.siteground.com/tutorials/phpmyadmin/create-populate-tables/
You need a login.php (where you log in), a test.php page (then you put in it whatever you want) and a check_User.php page (where to control if the credentials are correct).
Login.php
<html>
<head> <title>Login</title> </head>
<body>
<form action="check_User.php" method="post" id="login_form">
<label><b>Username</b></label>
<!-- USERNAME -->
<input type="text" placeholder="Enter Username" name="username" required>
<!-- PASSWORD -->
<label><b>Password</b></label>
<input type="password" placeholder="Enter Password" name="password" required>
<!-- LOGIN -->
<button type="submit">Login</button>
</form>
<body>
</html>
check_User.php
<?php
session_start();
$_POST["username"] = htmlspecialchars($_POST["username"]);
$_POST["password"] = htmlspecialchars($_POST["password"]);
$link = mysqli_connect("your_host", "your_db_username", "your_db_password", "your_db_name");
$query = "SELECT username, password
FROM your_db_name
WHERE username = \"".$_POST["username"]."\" AND password = \"".$_POST["password"]."\"
";
mysqli_query($link, $query);
$rows = array();
$result = mysqli_query($link, $query);
while($row = mysqli_fetch_array($result,MYSQLI_ASSOC))
$rows[] = $row;
/* correct match */
if(mysqli_affected_rows($link) == 1)
{
$_SESSION["username"] = $_POST["username"];
$_SESSION["password"] = $_POST["password"];
}
if(isset($_SESSION["username"]) && isset( $_SESSION["password"]))
header("Location:test.php");
else {
alert("Wrong username or password");
}
?>
test.php
<?php
session_start();
// not logged in, not showing this page
if((!isset($_SESSION["username"]) || !isset( $_SESSION["password"]))
header("Location:login.php");
?>
<html>
....whatever you want this page to do
</html>
