Module development and code questions

Upgrading from Drupal 8.9 to Drupal 9.2.7 and I've run into an issue with a custom module that was created by a past developer.  The error has to do with valid_email_address.  I see that it was deprecated and then removed according to this link (https://api.drupal.org/api/drupal/core%21includes%21common.inc/function/valid_email_address/8.8.x)

Hi All,

I was wondering if anyone has the react_social code. it is no longer available on github.

Thanks in advance.

I am upgrading a lot of inherited custom module projects to Drupal 8. In almost all of them, I am getting this error when scanning them with Drupal 9 upgrade status:

Class PHPUnit\Framework\TestCase not found.

I've Googled, and even found an accepted answer on StackOverflow.
But I am not able to figure out what to do.

My PHP version is 7.4.3.

I 've tried to install phpunit.

Hi, this is my first topic here and I hope have more clearly this doubt.

On D8 I could cast a field into a "orderBy" dynamic queries, but now in D9 I got an error, and I try to use other ways to CAST or CONVERT a field but I still got syntax errors...

I have this on my storage php file that is into a function I called from a Controller file:

$order_field is received from the Controller php file.

static function getSortedLimit($limit,$order_field) {...

Hello, 

So far I have used contributed modules for D7 & 8. I like to start learning D9 module development.

I am looking for some free courses just to get some basic knowledge before I purchase any paid course. 

Can you recommend me some guide, free and paid courses to learn D9 module development?

Thank you!

I have php block which displays links based on the combination of roles.