The get_template_part function is one of the most useful things available to WordPress theme developers. Although mostly used in themes for public, get_template_part is often used in custom WordPress websites as an alternative to the PHP include or require.

When using get_template_part with the Shortcode API, there are two things you should always keep in mind:

• get_template_part executes .php files which (most likely) generates output
• shortcode callback functions are expected to return a string and not generate any output

So when calling get_template_part within a shortcode callback function, you’ll see that all the output generated by get_template_part is output before the post content, and not replaced inline.

The solution is to use PHP’s output buffering. Create a buffer in your shortcode callback before running get_template_part, clear the buffer and return the content right after. Here’s a quick example with an ads shortcode, which can insert your theme’s ads.php file contents anywhere within a post or page:

function my_ads_shortcode( $attr ) {
    ob_start();
    get_template_part( 'ads' );
    return ob_get_clean();
}
add_shortcode( 'ads', 'my_ads_shortcode' );

The ob_get_clean() function stops buffering and returns whatever was output to the buffer after ob_start(). The same approach could be used with other functions and statement that generate output, such as include and require.

Pro tip: wp_cron() runs during the init action at the default priority, i.e. 10. If you’d like things to be available during your cron tasks, make sure you initialize them earlier: the init action at priority 9 and less, or other actions that run before init — beware of these, since other things might not be available at that time.

Also, Core Control is a good plugin to test and debug cron tasks.

WordPress sends e-mails in plain/text by default, and you can change that using the wp_mail_content_type filter. However, if you change the content type to text/html for all your e-mails, you might break some messages that contain links, because WordPress puts <angle brackets> around links.

From the RFC2396:

Using <> angle brackets around each URI is especially recommended as a delimiting style for URI that contain whitespace.

If you’re going to change the content type for all your e-mails issued by WordPress, don’t forget to rewrite the default messages with proper markup. You can do that with various other filters, like retrieve_password_message.

Finally, if you’re going to send e-mails in HTML, please make sure they all respond well to narrow screens on mobile devices, and preferably contain a plain/text version of the same message.