How can I protect my php application by licence key?

How can I protect my php application by licence key?

References

Reference NO 1

http://stackoverflow.com/questions/13658299/how-can-i-protect-my-php-application-by-licence-key

Reference NO 2

http://forums.codewalkers.com/php-applications-45/how-can-i-protect-my-php-application-by-licence-key-1180432.html

Reference NO 3

http://www.phpclasses.org/package/2298-PHP-Generate-PHP-application-license-keys.html

Reference NO 4

http://lowendtalk.com/discussion/9664/any-idea-about-protect-php-script-using-licence-key

Reference NO 5

http://www.gsdesign.ro/blog/how-to-protect-your-php-applications-for-free/

Reference NO 6

http://www.zend.com/en/products/guard/faq

Reference NO 7

https://easydigitaldownloads.com/docs/activating-license-keys-in-wp-plugins-and-themes/

Reference NO 8

http://wordpress.stackexchange.com/questions/37011/how-do-i-protect-my-premium-wordpress-app-theme-from-copying