Google Maps API for drawing polygons

https://developers.google.com/maps/documentation/javascript/examples/polygon-simple

Create WordPress Plugins with OOP Techniques

http://code.tutsplus.com/articles/create-wordpress-plugins-with-oop-techniques--net-20153

Google Maps APIs for Multiple Locations

https://www.taniarascia.com/google-maps-apis-for-multiple-locations/
http://wrightshq.com/playground/placing-multiple-markers-on-a-google-map-using-api-3/

Executing php inside a WordPress widget without any plugin

http://www.emanueleferonato.com/2011/04/11/executing-php-inside-a-wordpress-widget-without-any-plugin/

Implementing the Range Operator in PHP

http://www.sitepoint.com/implementing-the-range-operator-in-php/?utm_medium=email&utm_campaign=SitePoint%20PHP%20Newsletter%20%2008%20March%202016&utm_content=SitePoint%20PHP%20Newsletter%20%2008%20March%202016+Version+B+CID_7e9bb79efb7b4224dc833f91593527aa&utm_source=CampaignMonitor%20SitePoint&utm_term=Implementing%20the%20Range%20Operator%20in%20PHP

PHP's OPCache extension review

http://jpauli.github.io/2015/03/05/opcache.html

Create a custom plugin in wordpress

http://justintadlock.com/archives/2011/02/02/creating-a-custom-functions-plugin-for-end-users

http://wpcandy.com/teaches/how-to-create-a-functionality-plugin/#.Vt7skJN9634

http://code.tutsplus.com/tutorials/creating-a-custom-wordpress-registration-form-plugin--cms-20968