General developer forum

I'm having trouble adding a custom form element to my plugin. Essentially, it uses an existing element type, with custom HTML to be output before it. Here is a minimal example of my code:

local/my_plugin/classes/form/custom_text_input.php:

<?php
namespace local_my_plugin\form;

defined('MOODLE_INTERNAL') || die();
global $CFG;
require_once($CFG->libdir . '/form/text.php');

class custom_text_input extends \MoodleQuickForm_text
{    
    /**
     * Constructor
     */
    public function __construct($elementName=null, $elementLabel=null, $options=null, $attributes=null)
    {
        parent::__construct($elementName, $elementLabel, $options, $attributes);
    }

    /**
     * Returns HTML for the form element
     *
     * @return string
     */
    function toHtml()
    {
        $html = 'Custom text';
        $html .= parent::toHtml();

        return $html;
    }
}

local/my_plugin/classes/form.php:

<?php
namespace local_my_plugin;

use \html_writer;
require_once("$CFG->libdir/formslib.php");
class form extends \moodleform
{
    public function definition()
    {
        global $CFG;
        \MoodleQuickForm::registerElementType(
            'custom_text_input',
            "$CFG->dirroot/local/my_plugin/classes/form/custom_text_input.php",
            'local_my_plugin\form\custom_text_input'
        );
        $mform = $this->_form; // Don't forget the underscore! 
        $mform->addElement('custom_text_input', 'name', 'Test Element');
    }
}