如何在PHP中加载页面时放置ajax-loader

Help me guys to make page loader while the page is loading. I'm doing some paypal payment system. As I watch some pages, when the page is directing to paypal page, they put some loader(ajax loader) while the page is processing to redirect to "Paypal gateway". I'm also planning to put some Ajax loader in my other page while they're loading. So please help me guys. Here is my code.Help me guys to make page loader while the page