Drupal 7 Module Development

Drupal’s Theme Layer
Now, that was an extremely simple example, but when we start looking at more
complex render elements, we'll see that each array key in a render element can be
one of the following three things:
1. A render element property. These are prefixed by #.
2. A child element. All array keys not prefixed by # are considered to be
a child elements.
3. A variable to be passed to a theme function. In the render element these
variable's names are prefixed with # (just like properties are), but theme()
will strip the # from the name before sending it on to the actual theme
implementation.
Taking these slightly mush rules, we can examine the following render element:
$element = array(
'#prefix' => '',
'#suffix' => '!',
'kitten' => array(
'#type' => 'link',
'#title' => t('Kill me'),
'#href' => 'admin/core/hack',
),
'separator' => array(
'#markup' => '',
),
'domo' => array(
'#theme' => 'username',
'#account' => $account,
),
);
First, we should identify the children since they are the simplest to spot. kitten,
separator, and domo are the child elements of our render element. The separator
child element is another example of a simple #markup render element.
Looking at the domo element, we see that its #theme property is set to username.
drupal_render() will take that child element and pass it to theme() with a theme
hook of username; meaning that theme('username', $element['domo']) will be
called and theme() will strip the # characters from the front of all of the variables
before passing the data to theme_username().
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