Adds the Yoast SEO webmaster tools (and other sites) verification codes to all pages including the login page. This is helpful when running a membership site or similar that requires users being logged in to view the site.

yoast-webmaster-tools-verification.php

<?php

add_action( 'init', 'cameronjonesweb_yoast_front_page_head' );

function cameronjonesweb_yoast_front_page_head() {
	if ( class_exists( 'WPSEO_Frontend' ) ) {
		add_action( 'wpseo_head', array( WPSEO_Frontend::get_instance(), 'webmaster_tools_authentication' ), 90 );
		add_action( 'login_head', array( WPSEO_Frontend::get_instance(), 'webmaster_tools_authentication' ), 90 );
	}
}