How to use myhost.com/username in codeigniter?

Edit application/config/routes.php
add following line of code

$route['(.*)'] = "welcome/index/$1";  

Edit your welcome.php controller as follows

load->view('welcome_message');	
		echo "Username is ".$username;
	}	
}

you need to write .htaccess file for removing the index.php file from your url


  RewriteEngine On
  RewriteBase /cv_bank/

    #Removes access to the system folder by users.
    #Additionally this will allow you to create a System.php controller,
    #previously this would not have been possible.
    #'system' can be replaced if you have renamed your system folder.
RewriteCond %{REQUEST_URI} ^system.*
RewriteRule ^(.*)$ /cv_bank/index.php?/$1 [L]

    #When your application folder isn't in the system folder
    #This snippet prevents user access to the application folder
    #Submitted by: Fabdrol
    #Rename 'application' to your applications folder name.
RewriteCond %{REQUEST_URI} ^application.*
RewriteRule ^(.*)$ /cv_bank/index.php?/$1 [L]

    #Checks to see if the user is attempting to access a valid file,
    #such as an image or css document, if this isn't true it sends the
    #request to index.php
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /cv_bank/index.php?/$1 [L]




    # If we don't have mod_rewrite installed, all 404's
    # can be sent to index.php, and everything works as normal.
    # Submitted by: ElliotHaughin

    ErrorDocument 404 /index.php
   

you can now test in your browser by typing http://localhost/cv_bank/alex/. Here alex is username. You can see User name is alex in your browser.