Advanced custom field – posted fields from custom post type

You can find the Advanced Custom Fields fields which you added in the $_POST array under the ‘fields‘ key. Use ACF’s get_field_object() function to get the key of your field you added.

So if you named your field ‘nr_available’, you have to find its key to be able to find this field in the $_POST object (so you can fetch $_POST[‘fieldKey’]).

Check the following code:

function update_course($course_id)
{
    $nr_available = 0;
    $nr_registered = 0;

    $field1 = get_field_object('nr_available');
    $nr_available_key = $field1['key'];

    $field2 = get_field_object('nr_registered');
    $nr_registered_key = $field2['key'];

    // loop the $_POST['fields'] which contains all the Advanced Custom Fields fields which you added to the post you are editing
    foreach ($_POST['fields'] as $k=>$v)
    {
        // $k are the custom fields keys
        // $v are the custom fields values

        if ($k == $nr_available_key){
            $nr_available = $v;
        }
        if ($k == $nr_registered_key){
            $nr_registered = $v;
        }
     }

     // Do your checks after you get the values
     if ( ($nr_available > $nr_registered) || $nr_available <= 0 )
     {
        // Bad
        return;
     }
}
add_action( 'edit_post' , 'update_course');