Module development and code questions

What or where are the rules which refine whether I can call a function in another module or call it via module_invoke? Also does module_invoke allow variable references to be passed to the invoke module, which is to say changed in the module invoking the other module?

hi,

I have a question: there are two modules out there which support ecommerce stuff - the paypal framework + paypal subscription modules and the e-commerce thing.

functions I want to cover:

1. paypal payment
2. after buying via paypal the user can download a pdf file
3. after buying, the pdf should be available for the user, if he is a registered user
4. if he is not a registered user, he can still buy the pdf, but after login the pdf is not available for him

Maybe this is not the right question - so i will likely end up posting this a couple times here.
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What do i do to have a module put a piece HTML code ONCE into my page ?

I was trying to use hook_nodeapi which sort of works.. except that my page has 3 nodes on it.. therefore it was doing this 3 times... which in FF it didn't seem to be an issue (not sure why not.. likely it was doing it but other 2 copies were buried).. but they show up in IE.

Hi - Working in 4.7, upgrading some modules in my site.

I have a multi-select box, that seems to be throwing the newly updated:
"An illegal choice has been detected. Please contact the site administrator."

Error message being discussed in this issues post a couple weeks ago, I must be the "hacker henry" mentioned in the post :) http://drupal.org/node/45821

I have narrowed it down to this multi-select box, if a user tries to submit without having made a selection.

Just been at Drupal for a while and thought i would try designing my first module.

I had this idea of a "showcase" on another non-CMS site i did - and since i am moving that site over to Drupal figured i should give it a shot.

to start with ... check it out at: http://www.allanalog.com/d0214/

the SHOWCASE part hopefully is obvious - the porttion in my header that cycles through various blurbs on different technologies.

a little bit about it... in case anone is interested...

Maybe this is not the right question - so i wil llikely end up posting this a couple times here.
=================

What do i do to have a module put a piece HTML code ONCE into my page ?

I was trying to use hook_nodeapi which sort of works.. except that my page has 3 nodes on it.. therefore it was doing this 3 times... which in FF it didn't seem to be an issue (not sure why not.. likely it was doing it but other 2 copies were buried).. but they show up in IE.