Call to a member function get_row() on a non-object, Unable to use global $wbdp inside a plugin

Question

I am building a custom plugin for my WordPress site.

I made a php file called "checkrank" that obtains data from a custom table named "cranking".

$uid = $GET['id']; 

function checkExists($id){
    global $wpdb;
    $table_name = $wpdb->prefix . 'cranking';
    $exists = $wpdb->get_row( $wpdb->prepare( "SELECT * FROM $table_name WHERE id = %d" ) );

    $error = $wpdb->print_error();

    if(count($exists) > 0){
       echo 'Exists!';
    } else {
       echo 'Does not exist!';
    }

 return $error;
}

checkExists($uid);

The code is just to check whether the cranking table has the user with the provided ID.

The problem is, whenever i run this php file, i get this error :

PHP Fatal error: Call to a member function get_row() on a non-object

Where am I at fault here?
I think $wbdp is not initiated.

Answer 1

In a comment you clarify:

what i meant by running is going to the url
(mysite.com/wp-content/plugins/myplugin/checkrank.php?id=1)

In that case, you are running a standalone file.

Yes, one that resides inside WordPress' plugins folder, but you don't access it in the context of WordPress (i.e. from within it). Here, none of the WordPress core functions/methods/classes will be available, because they are simply never loaded.