is_plugin_active function doesn’t exist

I’m using WordPress 3.0.5 and have tested with 3.1rc4. In the main PHP file of my plugin, when I try to call is_plugin_active I get Call to undefined function is_plugin_active(). I can call add_action and add_filter. What should I check/change to fix this?

This is happening inside of the admin on the Plugins page. At the top of my main plugin file I have, if (function_exists('is_plugin_active')) { which always returns false.

I also can’t see the functions from my main plugin file in other plugins (if that helps any).

That’s because the file in which is_plugin_active() is defined – wp-admin/includes/plugin.php – is only loaded in the admin, after your plugin is loaded.

Thus, you can only call it after ‘admin_init’ has fired:

function check_some_other_plugin() {
  if ( is_plugin_active('some-plugin.php') ) {
    ...
  }
}
add_action( 'admin_init', 'check_some_other_plugin' );