How to develop an extension for a simple form post and post back? [closed]

I think the easiest way is adding the form via the Shortcode API. The code would look like this:

function myform_handler( $atts ){
    if (isset(@$_REQUEST['submit'])) {
        //Do something
    } else {
        $return = '<form action="" method="post">
                   <input type="text" name="myfield">
                   <input type="submit" name="submit>
                   </form>';
    }
    return $return;
}
add_shortcode( 'myform', 'myform_handler' );

Then you can add on any page the following Shortcode, that creates the form (and the form’s response):

 [myform]