How to store multiple values ​​in 1 column?

Question

MySQL

There is a category selection when adding material, it was necessary to make a multi -selection
in the form of writing

<select multiple="multiple" name="post_category[]" class="form-control" id="post_category">

through Debug it is clear that when submitting the form, the data is sent
category[2]
category[3]
But only 1 category is stored in the database category
table structure

Answer 1

You don't know what data normalization is. The design of relational databases requires that the data be as primitive as possible.
I recommend rethinking the structure of the database and reading this material.

Answer 2

1. You need to look at the data saving code.
Apparently, it is simply not provided for saving 2 or more categories
2. It is necessary to think about the data storage structure and data output.
Simply changing the html will not be able to assign material to multiple categories.