Use http:// or https:// in URLs based on URL of current page

common.php

@@ -0,0 +1,11 @@
+<?php
+function get_protocol() {
+	$protocol = "http";
+	if ( (!empty($_SERVER['HTTPS'] && $_SERVER['HTTPS'] !== 'off') ||
+		$_SERVER['SERVER_PORT'] == 443) )
+		$protocol = "https";
+	return $protocol."://";
+}
+$protocol = get_protocol();
+$base_url = $protocol.$_SERVER['HTTP_HOST'];
+?>