theme.php

<?php
/**
 * Retrieve the name of the highest priority template file that exists.
 *
 * Searches in the STYLESHEETPATH before TEMPLATEPATH so that themes which
 * inherit from a parent theme can just overload one file.
 *
 * @since 2.7.0
 *
 * @param array $template_names Array of template files to search for in priority order.
 * @param bool $load If true the template file will be loaded if it is found.
 * @param bool $require_once Whether to require_once or require. Default true. Has no effect if $load is false.
 * @return string The template filename if one is located.
 */
function locate_template($template_names, $load = false, $require_once = true ) {
	if ( !is_array($template_names) )
		return '';

	$located = '';
	foreach ( $template_names as $template_name ) {
		if ( !$template_name )
			continue;
		if ( file_exists(STYLESHEETPATH . '/' . $template_name)) {
			$located = STYLESHEETPATH . '/' . $template_name;
			break;
		} else if ( file_exists(TEMPLATEPATH . '/' . $template_name) ) {
			$located = TEMPLATEPATH . '/' . $template_name;
			break;
		}
	}

	if ( $load && '' != $located )
		load_template( $located, $require_once );

	return $located;
}