Freelance web developer
Yii 1 was released in 2008 and quickly established itself as a powerful MVC framework and a viable alternative to other PHP frameworks like CodeIgniter and later, Laravel. Version 1 of Yii had drawbacks though, such as poor environment config support, and the framework often needed a bit of wrapper code to get it to do what you wanted.
Yii 2 was released in 2014 as a completely re-written PHP framework which brought the framework up-to-date with new versions of PHP and resolved almost all of the previous version's shortcomings.
The developers of Yii announced in 2015 that Yii 1 would be deprecated, and currently Yii 1 is in maintenance mode only, with only security and PHP version fixes being made - and will no longer be supported by the developers from December 31st, 2020.
Many applications and websites built with Yii 1 still exist and will continue to function, even past the obsolecense date of the framework. However business owners who use Yii 1 to power their PHP web applications or website may want to consider upgrading in advance of the cut-off date in order to be certain of security standards and system availability.
Upgrading from Yii 1 to Yii 2 can be a complex task, as the framework is not backwards compatible - Yii 2 was designed from the ground up in a very different way. However upgrading from Yii 1 is perfectly possible and can often be a good opportunity to redesign and improve aspects of your software.
I have upgraded several websites and web applications to Yii 2 over the last few years and seen benefits to security, speed and maintainability of the software concerned.
For developers, the Yii website offers very good documentation on upgrading from Yii 1 to Yii 2.
For more information about upgrading from Yii 1 to Yii 2, please contact me: firstname.lastname@example.org.