[wp-hackers] Re: Odd behaviour when using classes and the register_activation_hook() function

John Blackbourn johnbillion+wp at gmail.com
Thu Dec 4 03:59:16 GMT 2008

Scrap that, this is a variable scope issue.

Any global variables that you want to reference inside the function
that is called by register_activation_hook() must be explicitly
declared as global inside the main body of the plugin (ie. _outside_
of this function). The plugin file is include()-ed inside another
function at the point where it is activated unlike at others times
when the plugin file is simply include()-ed. Phew. Bit of an odd one
to get your head around but there we go.

See @ http://wordpress.org/support/topic/201309

On Thu, Dec 4, 2008 at 2:25 AM, John Blackbourn
<johnbillion+wp at gmail.com> wrote:
> I've come across this odd issue and I'm not sure if I'm doing
> something wrong, so would appreciate any pointers.
> I am writing a plugin which uses register_activation_hook() to call a
> function when the plugin is activated. From within the function that
> gets called, another function is called which is a member function of
> a class defined in the plugin.
> I've cut the plugin down to the very minimum needed to demonsrate the
> code. See: http://johnblackbourn.com/temp/helloworld.phps .
> The problem I am seeing is that when the plugin is activated, this
> calling of the class member function is causing a fatal error.
> "Fatal error: Cannot redeclare activatehelloworld() (previously
> declared in [snip] )"
> Now, if we remove the line $helloworld->hello(); then all is fine and
> the plugin can be activated with no errors. If we instead call another
> function here that is not a class member function, all is fine. It
> appears that the act of calling a class member function at this point
> is doing something strange, or more likely, is acting in a manner that
> I don't understand.
> It might also be worth mentioning that, oddly enough, the plugin is
> actually activated despite this error being shown. This may be related
> or it may be a red herring.
> I'm stumped. I cannot see any reason why calling a class member
> function at this point would produce this error. Anyone?
> John.

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