Moodle form that is disabled based on customdata parameters

disable_moodle_form.php

<?php

require_once($CFG->dirroot.'/lib/formslib.php');

class my_form extends moodleform{

    function definition() {
    
    //check if the select must be disabled
    isset($this->_customdata['disabled_select']) 
        ? $disabled = 1 //use numeric booleans for this to work
        : $disabled = 0;

    //add a select
    $options = array('0' => 'first', '1' => 'second');
    $this->_form->addElement('select', 'my_select', 
                             'This is my select!', $options);
    //add the disabledIf rule (1 is disabled, 0 is enabled)
    $this->_form->disabledIf('my_select', $disabled);

    //add a submit button
    $this->_form->addElement('submit', 'save_changes', 'Save!');
    }
}