What you can do, is in the install code for the product, use something like:
PHP Code:
$file = file_get_contents('./showthread.php', FILE_USE_INCLUDE_PATH);
file_put_contents('./showthread_original.php', $file, FILE_USE_INCLUDE_PATH);
//Use the str_replace() or preg_replace() function(s) on the string $file to make your changes...then put the altered contents on the server.
file_put_contents('./showthread.php', $file, FILE_USE_INCLUDE_PATH);
And then in your product's uninstall code, you should restore the original file by getting the contents of the original you previously saved under the new name, then write that to the original name, and delete the file that stored the original contents:
PHP Code:
$file = file_get_contents('./showthread_original.php', FILE_USE_INCLUDE_PATH);
file_put_contents('./showthread.php', $file, FILE_USE_INCLUDE_PATH);
unlink('./showthread_original.php');