WordPress/wp-includes/template-loader.php

<?php
/**
 * Loads the correct template based on the visitor's url
 *
 * @package WordPress
 */
if ( wp_using_themes() ) {
	/**
	 * Fires before determining which template to load.
	 *
	 * @since 1.5.0
	 */
	do_action( 'template_redirect' );
}

/**
 * Filters whether to allow 'HEAD' requests to generate content.
 *
 * Provides a significant performance bump by exiting before the page
 * content loads for 'HEAD' requests. See #14348.
 *
 * @since 3.5.0
 *
 * @param bool $exit Whether to exit without generating any content for 'HEAD' requests. Default true.
 */
if ( 'HEAD' === $_SERVER['REQUEST_METHOD'] && apply_filters( 'exit_on_http_head', true ) ) {
	exit();
}

// Process feeds and trackbacks even if not using themes.
if ( is_robots() ) :
	/**
	 * Fired when the template loader determines a robots.txt request.
	 *
	 * @since 2.1.0
	 */
	do_action( 'do_robots' );
	return;
elseif ( is_feed() ) :
	do_feed();
	return;
elseif ( is_trackback() ) :
	include( ABSPATH . 'wp-trackback.php' );
	return;
endif;

if ( wp_using_themes() ) :
	$template = false;
	if ( is_embed() && $template = get_embed_template() ) :
	elseif ( is_404() && $template = get_404_template() ) :
	elseif ( is_search() && $template = get_search_template() ) :
	elseif ( is_front_page() && $template = get_front_page_template() ) :
	elseif ( is_home() && $template = get_home_template() ) :
	elseif ( is_post_type_archive() && $template = get_post_type_archive_template() ) :
	elseif ( is_tax() && $template = get_taxonomy_template() ) :
	elseif ( is_attachment() && $template = get_attachment_template() ) :
		remove_filter( 'the_content', 'prepend_attachment' );
	elseif ( is_single() && $template = get_single_template() ) :
	elseif ( is_page() && $template = get_page_template() ) :
	elseif ( is_singular() && $template = get_singular_template() ) :
	elseif ( is_category() && $template = get_category_template() ) :
	elseif ( is_tag() && $template = get_tag_template() ) :
	elseif ( is_author() && $template = get_author_template() ) :
	elseif ( is_date() && $template = get_date_template() ) :
	elseif ( is_archive() && $template = get_archive_template() ) :
	else :
		$template = get_index_template();
	endif;
	/**
	 * Filters the path of the current template before including it.
	 *
	 * @since 3.0.0
	 *
	 * @param string $template The path of the template to include.
	 */
	if ( $template = apply_filters( 'template_include', $template ) ) {
		include( $template );
	} elseif ( current_user_can( 'switch_themes' ) ) {
		$theme = wp_get_theme();
		if ( $theme->errors() ) {
			wp_die( $theme->errors() );
		}
	}
	return;
endif;