Grab the page id for the first page using a specific template. Ideal for situations with a template intended for a only one page.

get-page-id-by-template.php

<?php
/**
 * Get the first page id with the matching template slug.
 * 
 * @author Joshua David Nelson, [email protected]
 * 
 * @param string $template This is the template slug, like 'template-about-page.php'
 * 
 * @return mixed $page_id Int or false on failure
 */
if( !function_exists( 'get_page_id_by_template' ) ) {
	function get_page_id_by_template( $template ) {
		$args = array(
			'post_type' => 'page',
			'fields' => 'ids',
			'post_status' => 'publish',
			'nopaging' => true,
			'posts_per_page' => 1,
			'update_post_term_cache' => false,
			'meta_key' => '_wp_page_template',
			'meta_value' => esc_attr( $template ),
		);
		$pages = get_posts( $args );
		
		$page_id = isset( $pages[0] ) ? $pages[0] : false;
		
		return $page_id;
	}
}