evaled on page generation
 

What exactly does the part in bold mean, and is there a risk of variable name collision?

If you take a look at table datastore for record pluginlist you will find smth. linke this:

This is a serialized array.
Upon execution it will be unserialized:
Then this code will be executed:
Variable collisions are always possible.
As this unserializing and evaluating is being done at execution time it generates overhead. So for larger code modifications you might want to use include files - then less code must be unserailized and evaled.
Include files can also be pre-compiled/cached -> eAccelerator/ZEND Performance Suite/etc.

When are plugins first available, i.e., is a plugin first available in init.php or global.php or ???

I don't understand this question.

So, do you mean just use an include statement to not store so much stuff? By evaluating, do you mean just the plugin code is evaluated, or is it all included files?

Say I make a plugin that sets variable $foo - I am wondering when $foo is first available: a) by plugin and b) by hack.

EDIT: Argh, the Automerged Doublepost got me! :D

Only the include statement. Of yourse the included file must aso be parsed/compiled, but as already said files can be cached.

I don't understand this question, sorry.
What to you want to know - variable scope?
If your plugin is being called from within a function, the any variables you set (if you don't define them as global) will only be visible within this function.
If your plugin is being called from a global scope, then variables will be global.

Okay, thanks. Now when is $foo from a plugin first available, i.e., what is the first file that $foo will be able to be used in, if $foo is set via plugin?

$foo will be available after the plugin was called.

Right, but when is the plugin first called? I see some code in init.php, so are hooks/plugins available at the end of init.php but before global.php?

That depends on the hook you are using.
The first hook being called is fetch_userinfo from init.php (line 362):

ignore

Man, I don't get you.

Variables set in a Plugin will be available after the Plugin was called - and this time depepends on the hook.

Example
Variables set in a Plugin for hook forumdisplay_start will be avialable in code executed after forumdisplay.php line 98.

LOL, thanks for your patience, let me try to ask another way... :devious:

What is the first vB file where set_session_visibility() is available by plugin?

Every File.

Including at the top of init.php?

No. OK, to be more precise:
Every php file that is not an include-/class-file.
Or in other words as already said:

ignore

I give up. Maybe somebody else is able to explain it to you :(

got it. thanks for helping. :)