I did search the form and look into each menu of phpStrom, but I was not able to find out how to remove a project, that was added to an existing project.
Could anybody give me a hint?
What I did:
1. Opened a project
2. Then: File -> Open to select another project
3. Upon asking where to open it, I chose "in current window" and "append to existing project" (or similar ... writing from memory...)