Are there any materials on how to write a CRM system?

Question

PHP

There are some materials, guides on how best to write a CRM system, what is needed for this, etc.

Answer 1

Not claiming to be correct - personal experience.
The site is an internal product for work. There is a lot of data, you need to work with them in a certain way.
A typical page is selection filters and a table with data. Yes, some other interface solutions can be used to process them. Yes, a page of a specific client or graphics for a manual is a separate topic. However, most of the working pages are built exactly according to the "selection - table - work with its rows" scheme.
For a typical page, I took the trouble to write a mini-framework that builds the filter form and the results table according to the description in the form of an object with known fields. Most of the fields come from the backend - initial filter values, column headers, pagination data. Greatly reduces manual work... and gives the user a consistent interface. It also allows you to centrally add goodies like sorting by columns, final line, etc.
For requests, responses and other things, the built-in AJAX framework OctoberCMS is great. The data is conveniently represented by models, to which the same October allows you to literally add uploaded files in one line. If you need to give the admin the opportunity to dig into the data - add a description to the database tables, and they are displayed on the admin pages.
If someone wants to kick a bike - feel free ;)