How to associate phpstorm with multiple projects

This is the case. Suppose there are two projects, project A and Project B. Only one project can be opened in phpstorm each time. I open project A first, however, the source code of some classes in Project A is in Project B, but phpstorm is unknown. How can I associate the class in Project A with the source code in Project B? Yes... this is the case. Suppose there are two projects, project A and Project B;
Only one project can be opened in phpstorm each time. I open project A first, but the source code of some classes in Project A is in Project B, but phpstorm is unknown.
How can I associate the class in Project A with the source code in Project B?

Do not let me put Project A and Project B in A folder and open it in phpstorm. This is not the fundamental solution to the problem.

Thank you.

Reply content:

Suppose there are two projects, project A and Project B;
Only one project can be opened in phpstorm each time. I open project A first, but the source code of some classes in Project A is in Project B, but phpstorm is unknown.
How can I associate the class in Project A with the source code in Project B?

Do not let me put Project A and Project B in A folder and open it in phpstorm. This is not the fundamental solution to the problem.

Thank you.

Preference => project => directories can be configured with multiple directories. If both projects change frequently, we recommend that you

Another type is preference => language => php => include path. You can also add directories. If a project is not frequently modified (such as a class library or basic library), we recommend that you