Only consider user-defined functions as hooks

o man. hook #fail

will think about best path to victory for this one...

That certainly explains why I cannot find the date_diff() in the date module that I happen to have installed.

The only thing I can imagine doing is adding a more unique string to the _diff() hook, but that would break existing modules.

Is it possible to blacklist certain names for drupal hooks?

I will look into this as well as I would like to get this solved asap.

After looking around the PHP docs, this seems to be what we want to use:
http://us3.php.net/manual/en/function.get-defined-functions.php

PHP core functions get stored in the array called: internal
All others seem to get stored in the array called: user

The drupal_hook() needs to be changed to only call a hook if it is defined in the user array.

I have attached a patch that makes this check for drupal 6.x.

The patch changes module_invoke and module_invoke_all to have the following added:

<?php
$defined = get_defined_functions();

if (is_array($defined) && array_key_exists('user', $defined) && in_array($function, $defined['user'])){
  // function calls wrapped here
}
?>

I have moved this thread to drupal core because I believe this is the proper solution to this problem.
If you think otherwise, please move this issue back to the Diff project.

Bumping to D7 for consideration. Changing module_hook() seems to be the only thing necessary here.

I wonder what would happen if the get_defined_function() failed to return a properly populated array.

I am now thinking that if the get_defined_function() function failed to return sane results that the code should gracefully fail.
Would isset() be a better choice in this case than is_array($defined) && array_key_exists('user', $defined)?

For example:

$defined = get_defined_functions();

if ((isset($defined['user']) && in_array($function, $defined['user'])) || !isset($defined['user'])){
  // function calls wrapped here
}

The idea with that change would be to fallback to previous behavior if the $defined variable is not properly populated.

EDIT:
After reading #5, I just tried only changing module_hook(), but the original php error message appears.
Did you mean module_hook_all()?
I also just tried only changing module_hook_all() instead of module_hook() and the original php error message does not appear.

#4: drupal-6.x-avoid_internal_hookage-1.patch queued for re-testing.

This fixes the bug in Drupal 8

+++ b/core/includes/module.incundefined
@@ -1001,6 +1014,32 @@ function module_invoke_all($hook) {
+function module_function_user_defined($function) {
+  // This can't be cached as new files may be included at any time.
+  $defined = get_defined_functions();

This seems like it might be worth statically caching, thoughts?

This would very slow and a memory hog. Sorry.