Form Example 2

form_example_2.php

<?php

// This is your page function, where your content is rendered.
// $id is a 'page argument' (see Drupal hook_menu documentation)
function MYMODULE_page_function($id) {
  $output = '';
  
  // MYMODULE_get_values represents a function for a query that 
  // retrieves values from the database and accepts $id as an
  // argument to be used as a query condition.
  $values = MYMODULE_get_values($id);
  
  // Pass $values to your form
  $form = drupal_get_form('MYMODULE_example_form',$values);
  
  $output .= drupal_render($form);
  return $output;
}

function MYMODULE_get_values($id) {
  // Let's assume there is a query here returning this array:
  $values = array(
    'name' => 'Joe',
    'flavor' => 0,
  );
  
  return $values;
}

// Accept $values as an argument in your form function, but set
// it to NULL if nothing is passed.
function MYMODULE_example_form($form, &$form_state, $values = NULL) {
  // If $values is NULL, you can assume you're not editing a
  // set of values, but rather creating a new one.
  $edit = ($values == NULL) ? FALSE : TRUE;
  
  $form = array();
  
  $form['name'] = array(
    '#type' => 'textfield',
    '#title' => t("Your Name"),
    '#default_value' => $edit ? $values['name'] : NULL, // This is the key line!
  );
  
  $form['flavor'] = array(
    '#type' => 'select',
    '#title' => t("Favorite Flavor"),
    '#options' => array(
      0 => t("Vanilla"),
      1 => t("Chocolate"),
      2 => t("Aspertame"), 
    ),
    '#default_value' => $edit ? $values['flavor'] : NULL, // This is the key line!
  );
  
  // Change your submit to an update function if $edit is TRUE
  $form['submit'] = array(
    '#type' => 'submit',
    '#value' => t("Submit"),
    '#submit' => $edit ?  array('MYMODULE_example_update') : array('MYMODULE_example_submit'),
  );
  
  return $form;
}