When including template tags provided by a plugin or theme, it’s good practice to precede the tag with a conditional check to make sure that the function exists. For example: Instead of writing this:
echo usp_get_images();, we can write this:
if (function_exists("usp_get_images")) echo usp_get_images();. The first method works fine, but PHP will throw an error if the plugin is not installed or otherwise available. So to avoid the site-breaking error, the second method is preferred.