This snippet will check the current URL's protocol and if it matches http:// then it will redirect to the same URL but replace the protocol with https://

force-ssl.php

<?php

add_action( 'after_setup_theme', 'my_plugin_force_ssl' );

/**
 * Force unencrypted URLs to redirect to an SSL encrypted version.
 */
function my_plugin_force_ssl() {
	$protocol = stripos( $_SERVER['SERVER_PROTOCOL'],'https' ) === true ? 'https://' : 'http://';
	if( $protocol == 'http://' ) {
		$redirect_to = 'https://'. $_SERVER["HTTP_HOST"] . $_SERVER["REQUEST_URI"];
		wp_redirect( esc_url( $redirect_to ), 301 );
		exit;
	}
}