Before I install…

Yep, just do not lock your root .htaccess file or check if the script automatically unlocks the root .htaccess file, writes to it and locks it again. This is a very simple thing to do so if this script is not doing this then the code would be something like this.

$filename = ABSPATH . '.htaccess';
$stringReplace = @file_get_contents($filename);

if ( file_exists($filename) ) {
    // unlock the file first
    chmod($filename, 0644);
    // whatever code needs to be written here
   if ( file_put_contents($filename, $stringReplace) ) {
   // lock the file after writing to it
   chmod($filename, 0404);
   }
}

For the whatever code needs to be written section of the function above this is the simplest way to do a match and replacement. For “replace the landmark code with some new code” you would add the landmark code back into the root .htaccess file plus your new code so that the original root .htaccess file code is replaced and you additionally add your new code too.

// whatever code needs to be written here
$pattern = '/a landmark pattern match in the root htaccess file/';

if ( preg_match($pattern, $stringReplace, $matches) ) {
   $stringReplace = preg_replace('/the landmark pattern match code/', "replace the landmark code with some new code", $stringReplace);
}