根据会话名称php创建一个特定的文件夹

hi i have a website which has a login page and ten stores the login username as a session. once they have logged in i need them to press a button which then will create a folder with there username on it which has been gotten from the session. the problem is i can't get the name to show as the folder name as the folder gets created. the code is below.

<?php
  //Start the Session
session_start();

if (isset($_SESSION['username'])){
$username = $_SESSION['username'];
echo "Welcome " . $username . "
";
echo "This is the Members Area
";
echo "<a href='logout.php'>Logout</a>";
 
}else{
//3.2 When the user visits the page first time, simple login form will be displayed.
}
?>

<?php 
mkdir("$username"); 
?>

please help to name the folder which is created to be there username. i also would like if the folder is already there to be redirected to a different page to which the folder needs to be created.

</div>

Does the webserver user have access to write to the root directory? You should probably give a full path to a directory

//make sure the web server user (i.e. Apache) has write access
// to /var/www/users/ directory
mkdir("/var/www/users/$username");