Question

I want to echo ‘success’ if the variable is true. (I originally wrote “returns true” which only applies to functions.

$add_visits = add_post_meta($id, 'piwik_visits', $nb_visits, true);
if($add_visits == true){
         echo 'success';
}

Is this the equivalent of

$add_visits = add_post_meta($id, 'piwik_visits', $nb_visits, true);
if($add_visits){
         echo 'success';
}

Or does $add_visits exist whether it is ‘true’ or ‘false’;

Answer 1

Testing $var == true is the same than just testing $var.

You can read this SO question on comparison operator.
You can also read PHP manual on this topic.

Note: a variable does not return true. It is true, or it evaluates to true. However, a function returns true.