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A while ago I attempted to re-open previous work but could not get the XAMPP Control Panel to open, I guess because I re-installed Windows 10. I then decided to download the latest versions of XAMPP & CodeIgniter and then transfer my previous files to the new system.

Previous CI_VERSION = '3.1.1' - Latest CI_VERSION = '3.1.9' - Previous PHP version: 7.0.8 - Latest PHP version 7.2.3

When my current system directs to the "username" page which is where the username & password are created, I get the following error. This function was working OK in my previous system. It appears to be caused by a difference between the versions of PHP.

A PHP Error was encountered
Severity: Warning

Message: Use of undefined constant BASE_URL - assumed 'BASE_URL' (this will throw an Error in a future version of PHP)

Filename: username.php
Line Number: 47

This is Line 47

<form class="form" method="post" action="<?php echo BASE_URL . 'username'; ?>">

Backtrace:

File: C:xampphtdocsapplicationviewsusername.php
Line: 47
Function: _error_handler
This is Line 47 - ">

File: C:xampphtdocsapplicationcontrollersUsername.php
Line: 70
Function: view
This is Line 70 - $this->load->view('username', $data);

File: C:xampphtdocsindex.php
Line: 315
Function: require_once
BASE_URLusername">
This is Line 315 - require_once BASEPATH.'core/CodeIgniter.php';

Can somebody advise me of what I need to change?

Update 1

The only place I can find a definition of base_url is in /config/config.php which is following;

$config['base_url'] = 'http://localhost/';

And that has not been changed between my previous work and this current system.

Update 2

In the Controller Username.php there is a load function

$this->load->helper(array('form', 'url'));

But I can't find anything in the helper folder other than index file.

And again that has not been changed between my previous work and this current system.

The constant BASE_URL was defined somewhere in the old code but is not in the new. CodeIgniter does not define a constant called BASE_URL so it was done in some custom code. It probably looked something like this

define('BASE_URL', base_url());

There are other ways to obtain the value so the main thing to look for is define('BASE_URL',.

Likely places to look are customized versions of /config/config.php or /config/constants.php, but there are lots of other places including a base controller, custom library or some other user-created configuration file.

I couldn't find any constant called BASE_URL in the latest CodeIgniter source code.
Their documentation recommends using the base_url() function for returning the site's base URL.
https://www.codeigniter.com/user_guide/helpers/url_helper.html?highlight=base_url#base_url

There was, however, and older repo on GitHub where I could find a version of CodeIgniter where the constant BASE_URL was defined in the constants.php file.
https://github.com/assoft/codeigniter/blob/master/application/config/constants.php
Probably this was common practice for older versions.

So, in your example you can use the base_url() function. This should output http://www.example.com/username

<form class="form" method="post" action="<?php echo base_url('username'); ?>">

You have to import the url helper in the method of the controller corresponding to the view you're using.

$this->load->helper('url');

You should also fill in the value for the base url in the application/config/config.php file.

$config['base_url'] = 'http://www.example.com';

you have to define base_url. i am using this in header file.

 <script>
 var base_url = '<?php echo base_url(); ?>';
 </script>

I found my problem. In constants.php of my old system there is the following,

define('BASE_URL', 'http://localhost/');

I copied that into the my current contants.php file and it works, that is to say I don't get the error.