WordPress - template include file for theme from a location. Allows things like custom post types single-posttype.php to be in /myplugin/.

template-include.php

<?php

/**
 * Use a plugins template file, unless one exists in theme.
 * 
 * Useful when creating custom post types, allowing themes to create 
 * their own template.
 */
add_filter('template_include', function ( $template ) {
	if ( is_singular( 'location' ) ) { // `location` is a custom post type
        $theme_files     = array('single-location.php', 'myplugin/single-location.php');
        // check if exists in theme
		$exists_in_theme = locate_template($theme_files, false);
		if ( $exists_in_theme != '' ) {
		  return $exists_in_theme;
		} else {
          // no, than use plugins.
		  return plugin_dir_path(__FILE__) . 'single-location.php';
		}
	}
	return $template;
});