Php codes does not working

Hello

I am having a trouble about php code in my site:

When I use a code in my page, e.g <?php the_tags etc, a blank space is always added between < and ?, and the code does not works. I’ve tried to use several plugins for php codes, but nothing helps me. Someone know what is adding this blank space on my code? Someone can help me?

Please…

Thanks in advance.