Twig extension function read value - php

Question

I'm trying to create a set of simplified functions to let an user create a twig template that will be rendered as a form.
The form, when executed should read self generated data array.
The code below should print a field with name 'hello', when submitted (with example value 'world') create an array key hello and display the binded field with value 'world'.
Example template:
{{configkey('hello', 'Hello Field')}}
The extension function
$func = new Twig_SimpleFunction('configkey', function ($key='example', $label='label') {
echo '<div class="control-group">';
echo '<label class="control-label" for="' . $key . '">' . $label . '</label>';
echo '<div class="controls">';
echo '<input type="text" name="' . $key . '" value="{{config.' . $key . '}}"/>';
echo '</div>';
echo '</div>';
});
$this->_twigEnv->addFunction('configkey',$func);
the output

Answer 1

Thakns everyone for help but i found the solution easier than expected..
$func = new Twig_SimpleFunction('configkey', function ($key='example', $label='label', $value='') {
echo '<div class="control-group">';
echo '<label class="control-label" for="' . $key . '">' . $label . '</label>';
echo '<div class="controls">';
echo '<input type="text" name="' . $key . '" value="' . $value . '"/>';
echo '</div>';
echo '</div>';
});
The call will be
{{configkey('hello', 'Hello Field', config.hello)}}
Do you have a better aproach for the binding just using the key parameter ?