I had an idea today (that millions of others have probably already had) of putting all the sites script into a single file, instead of having multiple, seperate ones. When submitting a form, there would also be a hidden field called something like 'action' which would represent which function in the file would handle it.

I know that things like Code Igniter and CakePHP exist which help seperate/organise the code.

Is this a good or bad idea in terms of security, speed and maintenance?

Do things like this already exist that i am not aware of?