lock a page behind a password - php

Question

I'm trying to lock out a certain page to one user, with a username and a password. I tried a couple things, but what I think I need is a sort of login function. I have made logins before, so I used code of mine that I used before. For some reason however, it gives me an error that I coded in to know that the password is incorrect, even though it isn't. To make it a bit easier to see which specific error message is given, I've put in a 1 and a 2 behind the error, i get error number 1.
Below is my php code and below that is my html form I use.
$username = $password = "";
$username_err = $password_err = $login_err = "";
function pr($data, $kill_script = false)
{
echo '<pre>'.print_r($data,1).'</pre>';
if($kill_script) exit;
}
// Processing form data when form is submitted
if($_SERVER["REQUEST_METHOD"] == "POST"){
// Check if username is empty
if(empty(trim($_POST["username"]))){
$username_err = "Please enter username.";
}
else{
$username = trim($_POST["username"]);
}
// Check if password is empty
if(empty(trim($_POST["password"]))){
$password_err = "Please enter your password.";
}
else{
$password = trim($_POST["password"]);
}
// Validate Credentials
if(empty($username_err) && empty($password_err)){
//prepare a select statement
$sql = "SELECT username, password FROM user_confirm WHERE username = :username";
if($stmt = $conn->prepare($sql)){
// bind variables to the prepared statement as parameters
$stmt->bindParam(":username", $param_username, PDO::PARAM_STR);
//set parameters
$param_username = trim($_POST["username"]);
if($stmt->execute()){
// check if username exits, if yes then verify password
if($stmt->rowCount() == 1){
if($row = $stmt->fetch()){
$username = $row["username"];
$hashed_password = $row["password"];
if(password_verify($password, $hashed_password)){
// password is correct, so start new session
session_start();
//store data in session variables
$_SESSION["loggedin"] = true;
$_SESSION["username"] = $username;
//redirect user to welcome page
header("location: werk_edit.php");
}
else{
//invalid password, display error message
$login_err = "Invalid username or password. 1";
}
}
}
else{
//username doesn't exist, display error message
$login_err = "Invalid username or password. 2";
}
}
else{
echo "Something went wrong, please try again later.";
}
//close statement
unset($stmt);
}
}
//close connection
unset($conn);
}
HTML Form:
<?php
if(!empty($login_err)){
echo '<div class="alert alert-danger">' . $login_err . '</div>';
}
?>
<form action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]); ?>" method="post">
<label>Username</label>
<input type="text" name="username" class="form-control <?php echo (!empty($username_err)) ? 'is-invalid' : ''; ?>" value="<?php echo $username; ?>">
<span class="invalid-feedback"><?php echo $username_err; ?></span>
<label>Password</label>
<input type="password" name="password" class="form-control <?php echo (!empty($password_err)) ? 'is-invalid' : ''; ?>">
<span class="invalid-feedback"><?php echo $password_err; ?></span>
<input type="submit" name="login" class="btn btn-primary" value="Login">
</form>