显示名称为php时出错

Question

I get a undefined index on line 54 i get it when im not logged in. Here's line 54:

   <a class="btn btn-success" href="#">Welcome <?php echo htmlentities($_SESSION['user']['username'], ENT_QUOTES, 'UTF-8'); ?></a>  

Can somebody help making the logged out code of that?

<?php 
    require("common.php"); 
    if(empty($_SESSION['user'])) 
    { 
    echo 'Welcome guest, Login or Register';
    } 
?> 

Thats the logout but it gives off the error when logged i cant figure it out!

common.php

<?php 

// These variables define the connection information for your MySQL database 
$username = "****"; 
$password = "******"; 
$host = "*****"; 
$dbname = "users"; 

// UTF-8 is a character encoding scheme that allows you to conveniently store 
// a wide varienty of special characters, like ¢ or €, in your database. 
// By passing the following $options array to the database connection code we 
// are telling the MySQL server that we want to communicate with it using UTF-8 
// See Wikipedia for more information on UTF-8: 
// http://en.wikipedia.org/wiki/UTF-8 
$options = array(PDO::MYSQL_ATTR_INIT_COMMAND => 'SET NAMES utf8'); 

// A try/catch statement is a common method of error handling in object oriented code. 
// First, PHP executes the code within the try block.  If at any time it encounters an 
// error while executing that code, it stops immediately and jumps down to the 
// catch block.  For more detailed information on exceptions and try/catch blocks: 
// http://us2.php.net/manual/en/language.exceptions.php 
try 
{ 
    // This statement opens a connection to your database using the PDO library 
    // PDO is designed to provide a flexible interface between PHP and many 
    // different types of database servers.  For more information on PDO: 
    // http://us2.php.net/manual/en/class.pdo.php 
    $db = new PDO("mysql:host={$host};dbname={$dbname};charset=utf8", $username, $password, $options); 
} 
catch(PDOException $ex) 
{ 
    // If an error occurs while opening a connection to your database, it will 
    // be trapped here.  The script will output an error and stop executing. 
    // Note: On a production website, you should not output $ex->getMessage(). 
    // It may provide an attacker with helpful information about your code 
    // (like your database username and password). 
    die("Failed to connect to the database: " . $ex->getMessage()); 
} 

// This statement configures PDO to throw an exception when it encounters 
// an error.  This allows us to use try/catch blocks to trap database errors. 
$db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION); 

// This statement configures PDO to return database rows from your database using an associative 
// array.  This means the array will have string indexes, where the string value 
// represents the name of the column in your database. 
$db->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_ASSOC); 

// This block of code is used to undo magic quotes.  Magic quotes are a terrible 
// feature that was removed from PHP as of PHP 5.4.  However, older installations 
// of PHP may still have magic quotes enabled and this code is necessary to 
// prevent them from causing problems.  For more information on magic quotes: 
// http://php.net/manual/en/security.magicquotes.php 
if(function_exists('get_magic_quotes_gpc') && get_magic_quotes_gpc()) 
{ 
    function undo_magic_quotes_gpc(&$array) 
    { 
        foreach($array as &$value) 
        { 
            if(is_array($value)) 
            { 
                undo_magic_quotes_gpc($value); 
            } 
            else 
            { 
                $value = stripslashes($value); 
            } 
        } 
    } 

    undo_magic_quotes_gpc($_POST); 
    undo_magic_quotes_gpc($_GET); 
    undo_magic_quotes_gpc($_COOKIE); 
} 

// This tells the web browser that your content is encoded using UTF-8 
// and that it should submit content back to you using UTF-8
header('Content-Type: text/html; charset=utf-8'); 

// This initializes a session.  Sessions are used to store information about 
// a visitor from one web page visit to the next.  Unlike a cookie, the information is 
// stored on the server-side and cannot be modified by the visitor.  However, 
// note that in most cases sessions do still use cookies and require the visitor 
// to have cookies enabled.  For more information about sessions: 
// http://us.php.net/manual/en/book.session.php 
session_start(); 

// Note that it is a good practice to NOT end your PHP files with a closing PHP tag. 
// This prevents trailing newlines on the file from being included in your output, 
// which can cause problems with redirecting users.

Answer 1

For this answer, I'm assuming you have codes that correctly set the session variables.

<?php

 if(isset($_SESSION['user'])){

   $message = 'Welcome '.htmlentities($_SESSION['user'], ENT_QUOTES, 'UTF-8');

 }
 else { $message = 'Welcome guest, Login or Register'; }

?>


 <a class="btn btn-success" href="#"><?php echo $message; ?></a>