Upgrading a large Yii2 site?

Question

Yii

The bottom line is that there is a fairly large media project, it was launched and written 10 years ago, and the first version was written almost entirely procedurally. Then in the year of commercials in 2010 they began to update and transfer the functionality to yii1. Now a decision has been made to complete the site and introduce new functionality along the way of redesign. The question arises that the site needs to be redone taking into account modern realities and technologies, respectively, throwing out the old part, rewriting everything in advance on yii2. Due to the fact that it will not be possible to rewrite all the functionality at once, I would like to leave the old part working and slowly roll the new functionality into yii2.
The catch is that the old part of the site works on php5.3, and even with globals enabled (don't swear, I didn't write it). So how can you update all of this? Considering that yii2 works on 5.4
UPD: as nonsense. Move the old site to another directory and run it only locally. Launch a new site on yii2 when passing the request, check if this functionality is implemented, if not, then as an option, execute a request to the local site through file_get_content, for example, localsite/path/to/need/page.html, and display the result in the browser to the user.

Answer 1

Rewrite the site again, after all, it was written 10 years ago, once every 10 years at least you can (preferably) update the site completely.
With fallbacks there will be porridge.

Answer 2

Write acceptance-functional tests
Put two projects side by side
Create two php-fpm pools
Push projects through URLs in nginx

Answer 3

Rewrite again.
Here is some information on upgrading from Yii1 to Yii2 that might come in handy.