What potential pitfalls should be considered when allowing users to choose a storage location on a web server in PHP?

When allowing users to choose a storage location on a web server in PHP, potential pitfalls include security vulnerabilities such as directory traversal attacks, where users can access sensitive files outside of the intended storage location. To mitigate this risk, it is important to validate and sanitize user input to ensure that the chosen storage location is within the allowed directory. Additionally, setting proper file permissions and restricting access to sensitive directories can help prevent unauthorized access.

// Validate and sanitize user input for storage location
$storageLocation = '/path/to/default/directory/';

if(isset($_POST['storage_location'])){
    $userInput = $_POST['storage_location'];
    $userStorageLocation = realpath($storageLocation . $userInput);

    // Check if the user storage location is within the allowed directory
    if(strpos($userStorageLocation, $storageLocation) === 0){
        // Proceed with storing files in the user's chosen location
    } else {
        // Display an error message and do not allow access
        echo "Invalid storage location.";
    }
}