How to open a file from the integrated terminal in Visual Studio Code?

If you are having command not found: code in MacOS, use a full path to it.

/Applications/Visual\ Studio\ Code.app/Contents/Resources/app/bin/code -r [filename]

just

code file_name

I tried it on Win10 and on Linux (Ubuntu)

You can use -r or --reuse-window command line option.

code -r something.php

I don't know what operating system you're using, but on MacOS you can just say open filename.ext in the integrated terminal, and it will open a new tab in the same VSCode instance, ready for you to edit.