Formatting HTML output of theme function

I've implemented my own theme function (开发者_如何学运维theme_my_theme_function) and have implemented hook_theme to tell Drupal about my function. Everything works correctly and I get the output I want on the page, except...

When I look at the HTML source code, all the HTML code outputted by my function is all in one long line! How do I get my theme function to output formatted HTML that is neat and easy to read?

---

If you want the HTML to be formatted, you're going to have to do it yourself when you build the HTML strings. The easier way to do that is to use a template file as opposed to building strings of HTML manually (remember that PHP is essentially a HTML template language). There is a pretty good write up of how to do that here: http://drupal.org/node/165706

The core node module has a pretty good example:

node.module:

<?php

/**
 * Implementation of hook_theme()
 */
function node_theme() {
  return array(
    'node' => array(
      'arguments' => array('node' => NULL, 'teaser' => FALSE, 'page' => FALSE),
      'template' => 'node',
    )
  );
}

node.tpl.php:

<div id="node-<?php print $node->nid; ?>" class="node<?php if ($sticky) { print ' sticky'; } ?><?php if (!$status) { print ' node-unpublished'; } ?> clear-block">

<?php print $picture ?>

<?php if (!$page): ?>
  <h2><a href="<?php print $node_url ?>" title="<?php print $title ?>"><?php print $title ?></a></h2>
<?php endif; ?>

  <div class="meta">
  <?php if ($submitted): ?>
    <span class="submitted"><?php print $submitted ?></span>
  <?php endif; ?>

  <?php if ($terms): ?>
    <div class="terms terms-inline"><?php print $terms ?></div>
  <?php endif;?>
  </div>

  <div class="content">
    <?php print $content ?>
  </div>

  <?php print $links; ?>
</div>

There may also be a way to integrate HTMLTidy into Drupal so that it "beautifies" the markup before it's output, but I've never done that myself.

Ultimately, I would highly recommended not worrying about the formatting of your HTML output. Instead, use Firebug for Firefox or Inspector for Chrome/Safari. These both have an "Inspect Element" tool that lets you view the markup of the page in a nice browsable, editable tree. It's invaluable for web development.

*EDIT* theme_item_list does minimal formatting of HTML output. Here is an example of a list generated by theme_item_list:

<div class="item-list"><ul><li class="first">1</li> 
<li>2</li> 
<li>3</li> 
<li class="last">4</li> 
</ul></div>

In the code for theme_item_list you can see that it just adds a "\n" after the <li>:

$output .= '<li' . drupal_attributes($attributes) . '>' . $data . "</li>\n";