Adding property in all Contrellers: MY_Controller vs BaseController

It's a matter of personal preference. The one possible reason to go with the BaseController is you can still extend CI_Controller without bringing the $controller_url property along if needed.

About the code. You have one syntax error - you're failing to set the class property in the constructor and are setting a local variable instead.

PHP Code:

$controller_url = config_item('base_url') . $this->uri->segment(1); 


should be

PHP Code:

$this->controller_url = config_item('base_url') . $this->uri->segment(1); 


In case you're interested there are other ways to get the controller.
Here's one way that's a tiny bit more efficient than calling $this->uri->segment(1)

PHP Code:

$this->controller_url = config_item('base_url') . $this->router->class; 


Or, you could use the PHP magic constant __CLASS__

PHP Code:

$this->controller_url = config_item('base_url')  . __CLASS__; 


Or a not so efficient but probably more robust

PHP Code:

$this->controller_url = $this->config->base_url($this->router->class);