remove user_meta data from database for all users

I’d like to create a button for admins to be able to delete from table wp_usermeta -> key: ‘ref_credit’ for all users.

Is there simple resource friendly way to do this? does wp have built in function for this?

I found this: http://codex.wordpress.org/Function_Reference/delete_user_meta

But this is only for single user.

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A simple example would be to get all users for a specific role, iterate over the returned results and apply the delete_user_meta function for the given meta_key. Stir and let simmer for a fraction of a second and all user meta for that key shall be gone.

function say_goodby_to_the_meta(){

    $role = 'subscriber';
    $users = get_users('role='.$role);

    foreach ($users as $user) {

        delete_user_meta($user->ID, 'ref_credit');

    }

}  

This example is void of any error checking or return response but at its core this will do what you what you need. Associate the function with the click of a button or conditionally call and run this function in plugin or theme files based upon your use-case.

Taking the above function one step further and associating it with the click of a button within the dashboard you can add this function to admin_init hook.

add_action('admin_init', 'say_goodby_to_the_meta');

function say_goodby_to_the_meta(){

    //check if admin, if not, lets get out of here - don't run remainder of function 
    if ( !current_user_can('activate_plugins') ) 
        return;

    //check that $_POST is not empty & that correct wp_nonce is supplied
    //checl that $_POST['delete_credit'] is also set and passed with $_POST variables
    if ( !empty($_POST) 
         && check_admin_referer('delete_key','nonce_name') 
         && isset( $_POST['delete_credit'] ) ) 
     {
         //set your role to apply this action to
         $role = 'subscriber';

         //get all users of this role (i.e. Subcribers)
         $users = get_users('role='.$role);

         //iterate of each user returned applying our delete_user_meta function
         foreach ($users as $user) {
             delete_user_meta($user->ID, 'ref_credit');
         }

     } else {

         //return your error conditions here if you like or do further processing
         echo "I say I don't even know you, I say that you're not authorized,\n
               I don't understand you, so why do I judge your vars?\n
               Armand Van Administrator";

     }
}  

This assume you are using a wp_nonce_field in your form and that your form also contains $_POST['delete_key'] as one of the submitted $_POST vars, whether thats a hidden input or not. Calling check_admin_referrer is a convenient way to check whether your request has originated from an administrative page in the dashboard.