Using current_user_can() inside a plugin causes a fatal error

[PhilippH]

I experienced that using the function current_user_can() causes an fatal error:
Fatal error: Call to undefined function wp_get_current_user() in /var/www/wp-includes/capabilities.php on line 1281

For reproducing the error:

• Download wordpress 3.5.1 and install it.
• Install LeagueManager plugin (​http://wordpress.org/extend/plugins/leaguemanager/)
• Activate it

or

• add following line at the end of wp-content/plugins/hello.php:

  echo current_user_can("switch_themes");
  // any capability causes the error
  

My environment:

• Debian wheezy
• apache2 & php5 & mysql from official package mirror

I also noticed that this error is caused by using user_can() inside a plugin. (with an other error message indeed)
After adding include("pluggable.php") to capabilities.php, the problem seems to be resolved!

The line to add to wp-content/plugins/hello.php shoud be:
echo current_user_can("switch_themes");

The current_user_can() function, along with all the others in pluggable.php, are pluggable (hence the file name). This means they are explicitly defined after plugins are loaded so that plugins may override them (for example to implement a different authentication mechanism).

This means you cannot use any of the functions in pluggable.php directly in a plugin file (which you shouldn't do anyway). You must use them inside functions which are called on hooks.

For example:

function my_special_function() {
  if ( current_user_can( 'do_whatever' ) )
  // do your thing
}
add_action( 'plugins_loaded', 'my_special_function' );

(A note, current_user_can() isn't pluggable, but it requires wp_get_current_user(), which is.)