Well, you should at least post your attempts at coding this, so people can comment on them and help you better.
do you know how to use regular expressions? Do you want both the <body> and </body> tag removed or just the opening one?
echo preg_replace("/(<body>)*/", $replacement, $subject);
that will replace all ocurances of <body> in $subject with $replacement. I think that's what you wanted?
to replace both the end tag and the starting tag, there's prolly a way you can do it with one pattern, but I'd just use two cause i'm no expert.
echo preg_replace(array("/(<body>)/", "/(</body>)/"), $replacement, $subject);
but you should be looking at http://www.php.net/manual/en/function.preg-replace.php
although it doesn't really give much of an indication of the syntax for reg exp patterns. I learn all of them from a perl website.
Have a look around the net for more info.
cya man,
-Adam 🙂