Adding php code to a page just saves it as ‘text’, not ‘code’

hey ya’ll – So I’m trying to add a bunch of php to a new page in wordpress 2.5.1, but when i save and preview it, it just displays all the text, like ‘<?php echo “test”; ?>’ as opposed to just ‘test’

is there some option i need to turn on to not treat stuff as actual text like that? thanks!