register_activation_hook DOES NOT WORK

After long struggle with it I found why.
1. register_activation_hook() must be called from the main plugin file – the one that has “Plugin Name: …” directive.
2. Your hook function must be in the same file as well. From that function it is ok to call other functions defined in other files.
3. Doing echo “My hook called!”; – does not work from it. So this is not a good way to judge whether it working or not.
4. Your global variables must be explicitly declared as global for them to be accessed from inside of my_activation_hook().
Here’s a tiny plugin code sample with working code to demonstrate all that:

<?php
/*
Plugin Name: A Test
Description: A Test
*/

require_once (dirname(__FILE__) . '/my_other_file.php');

// This code *will not* work. Activation hook function must be defined in the main plugin file.
//    register_activation_hook (dirname(__FILE__) . '/my_other_file.php', 'my_other_function');

// This code will work.
register_activation_hook (__FILE__, 'test_activated');

// This is correct way to declare/access globals.
global $some_var;    // globals must be declared explicitly. Without this you will not be able to access '$some_var' from within 'test_activated()' function.
$some_var = 'hey';

//===========================================================================
function test_activated ()
{
   global $some_var; // Now it will be 'hey'.

   // This function is defined in 'my_other_file.php'
   my_other_function ();

   // This will not work, so don't try. If you need logging write something in temporary log file in here via fopen/fwrite.
	// If you want to quickly test if your activation hook works - put exit() into it. At least you'll see error during activation.
   echo 'test_activated called!';
}
//===========================================================================

?>

Gleb Esman

Hi jimrowe,

I’m having an identical problem. I haven’t found a solution yet. Have you?

If I find one I’ll let you know.

Cheers,

Brent

You can test if a function exists before defining it:

<?php
if (!function_exists('test_activated')) {
  function test_activated() {
    // your activation code
  }
}

tom.

maybe you can try with this
register_activation_hook (plugin_basename(_FILE_), your function)

OK, I figured out the “Fatal error: Cannot redeclare class” problem.

My activation hook function was a static method on the main plugin class:

register_activation_hook(__FILE__, 'MyPluginClass::activate');

In this function I included another file. In this other file, I used the $this variable mistakenly forgetting it was a static function so there was no $this. If I get the class instance as I should, for instance from a global, the error goes away.

In any case this is a bizarre and misleading error meaning there is an error in the activation hook.

If you follow the instructions on the http://codex.wordpress.org/Creating_Tables_with_Plugins page, there are two issues with it:

1) In the “The Whole Function” section, the first line of the code block needs to explicitly declare $jal_db_version as global. So the very first line of the block should be:
global $jal_db_version;

2) The dbDelta function causes a fatal error – the now infamous “cannot redeclare function xxxx” error that so many folks are having trouble with when trying to use this code. Remove this line:
dbDelta($sql);
and replace it with:
$wpdb->query($sql);
and the code will work properly.

NOTE: I’m using WP 3.0