PHP 'inject html' question

[spliff]

can i put a PHP code on 1 page that injects a certain HTML code in whatever page is displayed in the iframe on that page? (position is not very important)

if yes, how?
if no, tell me! :P

[NightShift58]

str_replace() (http://www.php.net/manual/en/function.str-replace.php)

If the search & replace is of the more complex kind, you'll have to wait for the gentleman from Alicante to read this...