This is an example Symfony form event listener.

symfony-form-event-listener.php

<?php
    $builder->addEventListener(FormEvents::POST_SUBMIT, function ($event) {
       $form = $event->getForm();
       $oldPassword = $form->get('old_password')->getData();
       $newPassword = $form->get('password')->get('first')->getData();
 
       $user = $this->security->getUser();
 
       if (!$this->passwordEncoder->isPasswordValid($user, $oldPassword)) {
          $form->get('old_password')->addError(new FormError('The old password is incorrect.'));
       }
 
       if ($this->passwordEncoder->isPasswordValid($user, $newPassword)) {
          $form->get('password')->addError(new FormError('The new password should be different from the old password.'));
       }
   });