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Sure, here's an example code for a simple login page using PHP PDO:

index.php:

PHP:
<?php
session_start();
if(isset($_SESSION["loggedin"]) && $_SESSION["loggedin"] === true){
    header("location: welcome.php");
    exit;
}
require_once "config.php";
$username = $password = "";
$username_err = $password_err = "";
if($_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)){
        $sql = "SELECT id, username, password FROM users WHERE username = :username";
        if($stmt = $pdo->prepare($sql)){
            // Bind variables to the prepared statement as parameters
            $stmt->bindParam(":username", $param_username, PDO::PARAM_STR);
            // Set parameters
            $param_username = trim($_POST["username"]);
            // Attempt to execute the prepared statement
            if($stmt->execute()){
                // Check if username exists, if yes then verify password
                if($stmt->rowCount() == 1){
                    if($row = $stmt->fetch()){
                        $id = $row["id"];
                        $username = $row["username"];
                        $hashed_password = $row["password"];
                        if(password_verify($password, $hashed_password)){
                            // Password is correct, so start a new session
                            session_start();
                            // Store data in session variables
                            $_SESSION["loggedin"] = true;
                            $_SESSION["id"] = $id;
                            $_SESSION["username"] = $username;
                            // Redirect user to welcome page
                            header("location: welcome.php");
                        } else{
                            // Display an error message if password is not valid
                            $password_err = "The password you entered was not valid.";
                        }
                    }
                } else{
                    // Display an error message if username doesn't exist
                    $username_err = "No account found with that username.";
                }
            } else{
                echo "Oops! Something went wrong. Please try again later.";
            }
            // Close statement
            unset($stmt);
        }
    }
    // Close connection
    unset($pdo);
}
?>
 
<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>Login</title>
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.css">
    <style type="text/css">
        body{ font: 14px sans-serif; }
        .wrapper{ width: 350px; padding: 20px; }
    </style>
</head>
<body>
    <div class="wrapper">
        <h2>Login</h2>
        <p>Please fill in your credentials to login.</p>
        <form action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]); ?>" method="post">
            <div class="form-group <?php echo (!empty($username_err)) ? 'has-error' : ''; ?>">
                <label>Username</label>
                <input type="text" name="username" class="form-control" value="<?php echo $username; ?>">
                <span class="help-block"><?php echo $username_err; ?></span>
            </div>    
            <div class="form-group <?php echo (!empty($password_err)) ? 'has-error' : ''; ?>">
                <label>Password</label>
                <input type="password" name="password" class="form-control">
                <span class="help-block"><?php echo $password_err; ?></span>
            </div>
            <div class="form-group">
                <input type="submit" class="btn btn-primary" value="Login">
            </div>
            <p>Don't have an account? <a href="register.php">Sign up now</a>.</p>
        </form>
    </div>    
</body>
</html>

config.php:

PHP:
<?php
// Database credentials
define('DB_SERVER', 'localhost');
define('DB_USERNAME', 'username');
define('DB_PASSWORD', 'password');
define('DB_NAME', 'dbname');
// Connect to database
try{
    $pdo = new PDO("mysql:host=" . DB_SERVER . ";dbname=" . DB_NAME, DB_USERNAME, DB_PASSWORD);
    // Set the PDO error mode to exception
    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
} catch(PDOException $e){
    die("ERROR: Could not connect. " . $e->getMessage());
}
?>

Note that you would need to replace the database credentials with your own in config.php. Also, you would need to create a users table in your database with columns id, username, and password.
 

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