This works like a charm, fast and accurate:
function replace_string_in_file($filename, $string_to_replace, $replace_with){
$content=file_get_contents($filename);
$content_chunks=explode($string_to_replace, $content);
$content=implode($replace_with, $content_chunks);
file_put_contents($filename, $content);
}
Usage:
$filename="users/data/letter.txt";
$string_to_replace="US$";
$replace_with="Yuan";
replace_string_in_file($filename, $string_to_replace, $replace_with);
// never forget about EXPLODE when it comes about string parsing // it's a powerful and fast tool