Do you need to be online when writing and running php scripts or code?

Answer 1

PHP needs to go through the PHP parser. This is commonly done through a module or CGI for a webserver.

All you need to work with PHP is a webserver with PHP installed. If the server is somewhere on the internet, yes you need to be online. But you can always run a server on your own computer. Look into Apache HTTP Server, and then install PHP on it.

That's pretty much what a developer does anyway.

Answer 2

If you download and run a copy of a server, like say, the free Apache server, then you can run php on yer own computer. Instructions are in the cited texts.