相关疑难解决方法(0)

以下if条件导致错误:

if( !empty ($this -> session -> userdata( 'user_id') ) )
{
    $user_id = $this -> session -> userdata( 'user_id' );
} 
else 
{ 
    $error = "ID doesn't exist";
}

我收到以下错误:

Fatal error: Can't use method return value in write context in (line number)

我该如何解决？