PHP Keeping URL the same.

[Gunner54]

<FORM METHOD=POST ACTION="YAY.php" ACTION="index.php">
<CENTER><INPUT NAME="Register" VALUE="YAY" TYPE="submit"></CENTER>
</FORM>

when you click on yay the url turns from :

www.site.com/index.php

into www.site.com/YAY.php

how can i keep the url www.site.com/index.php BUT change the page into YAY.php

Thank you.