Customising the footer of the WordPress admin can be useful when you want to maintain your brand presence on a client's website.
The following code snippet will replace the default WordPress admin footer with your own HTML content.
add_filter( 'admin_footer_text', 'custom_admin_footer_text' );
function custom_admin_footer_text( $default_text )
{
return '<span id="footer-thankyou">Website developed by <a href="https://www.jnorton.co.uk">Justin Norton</a><span> | Powered by <a href="https://www.wordpress.org">WordPress</a>';
}