How to debug user_register hook

Question

I created this selectbox inside my user-edit.php screen trough edit_user_profile hook.

<table class="form-table">
    <tbody>
        <tr>
            <th>
                <label for="select_membership">Plano de Membros</label>
            </th>
            <td>
                <select id="select_membership" name="select_membership"><option id="0">Sem plano</option>
                    <option value="1">Plano 1</option>
                    <option value="2" selected="selected">Plano 2</option>
                    <option value="3">Plano 3</option>
                    <option value="4">Plano 4</option>
                    <option value="5">Plano 5</option>
                    <option value="6">Plano 6</option>
                    <option value="7">Plano 7</option>
                </select>
            </td>       
        </tr> 
    </tbody>
</table>

Now I'm trying to get the selectbox value inside the user_register hook with this function:

public function update_membership_field( $user_id ){
        $membership_methods = new IMS_Membership_Method();
        $membership_methods->update_user_membership($user_id, $_POST['select_membership']);
    }

But nothing happens. How I can know if some data is passing through this function to check if something is happening? How to debug? I can't print nothing using this function.

Answer 1

Rafael, (as requested, turning comments into answer)

'user_register' fires when a user registers, not when a user is edited or updated. Try something like https://codex.wordpress.org/edit_user_profile_update

Wrt output in hooks. It depends when/how the hook is being executed. The user profile ones are tricky because wp does stuff to prevent a double submission. One could use error_log to output where one can read it. More detail here: Debug output during filter execution doesn't work…

This might also help to understand https://stackoverflow.com/questions/7878279/… and for plugin specifc logging How to log plugin errors to plugin error_log file