By default, the decimal separator in php (also in mysql) is a dot (.).
So when we work with floats in Yii (in calculations, validation, sql statements etc.), the decimal separator has to be a dot.
If we want to use for example a comma (,) as the decimal separator, that is if we want to display numbers and enable users to enter numbers with a comma before the decimals, we have to...
Uploading files in a webapp can be extremely tricky and sometimes the quickest way to do it is to store the file directly in the webserver or into a DNS (like Amazon S3) and then to save the link and the metadata inside a table in the DB. The thing is that you'll have to deal with file permissions, server storage, file management and so on (which is perfectly fine, by the way).
With the adoption of PJax on Yii2 things have change quite a bit with GridView when it comes to work with them in AJAX mode. It will probably be confusing at the beginning, but then you will soon realize how powerful the new approach is.