force-ssl.php

// FORCE ALL TRAFFIC TO USE SSL

add_filter( 'admin_init' , 'prefix_add_force_ssl_setting' );

add_action('template_redirect', 'prefix_force_ssl');

function prefix_add_force_ssl_setting() {

	register_setting( 'general', 'prefix_force_ssl', 'esc_attr' );

	add_settings_field('prefix_force_ssl', 'Force SSL' , function() {

		echo '

			<fieldset>

				<legend class="screen-reader-text"><span>Force SSL</span></legend>

				<label for="prefix_force_ssl">

					<input type="checkbox" name="prefix_force_ssl" id="prefix_force_ssl" value="1" ' . checked('1', get_option('prefix_force_ssl'), false) . ' />

					Redirect all users to SSL versions of pages.

				</label>

				<p><i>(Only check this if you have a valid SSL certificate installed!)</i></p>

			</fieldset>

		';

	} , 'general' );

}

function prefix_force_ssl() {

	if ( ! is_ssl() && get_option('prefix_force_ssl') == '1' ) {

		wp_redirect('https://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'], 301 );

		exit();

	}

}

Add this to a theme's functions.php file, to give your theme an option in the Settings->General page. If checked, all traffic will be redirected to an https version of the requested page.

Obviously this requires that your server will serve over https, which requires you have a valid SSL certificate.