Set a default value to a widget if it is not setted in a form (PHP Symfony)

You can alter a form's submitted data in the bind() method. Use this is if you want to change the behaviour of the form depending on what the user has selected/submitted.

If you want to alter how the object is altered after form data has been validated and ready to be persisted to the database, then take a look at the updateObject() method.

class myForm
{
    public function configure()
    {
        // configuration
    }    

    public function bind($taintedValues = array(), $taintedFiles = array())
    {
        // I can alter the behaviour of the form here, depending on the data submitted
        if (isset($taintedValues['do_not_store_my_personal_details'])) {
            // Change the value of a form field. If the form doesn't pass validation, the user will see any information entered into the phone_number fields has been deleted
            $taintedValues['phone_number'] = null;
        }

        return parent::bind($taintedValues, $taintedFiles);
    }

    public function updateObject($values = null)
    {
        if ($values === null) {
            $values = $this->getValues();
        }

        // Override the data stored with the object
        if ($values['do_not_store_my_personal_details'] == true) {
            $values['phone_number'] = null;
        }

        return parent::updateObject($values);
}