I\'d just be worried about throwing up errors if i ran that. It only checks if the get field is set once. The way to do this without incurring errors would be to have all the checks for the pages inside the \"if (isset($_GET[\'page\'])){\". I\'d also suggest using switch instead of \"if\" statements for the page numbers.
just a suggestion, but to decrease the size of the script, consider sticking the search and replacements into arrays. also, just using str_replace (or as Hawkee suggested, str_ireplace), means that you aren\'t actually validating the input, meaning that the user could possibly break up the format of the output page by adding too many closing or opening tags. A preg_replace would be better, that way if there are any mismatches, they aren\'t replaced at all.