NEW: 
function add_text_to_file($file, $text) {
	if(file_exists($file) and is_writable($file)) {
		$filecontents = file($file);
		$fout = fopen($file, "w");

		$filecontents = array_map('rtrim', $filecontents);
		array_push($filecontents, $text);
		$filecontents = array_unique($filecontents);

		$file_text = implode("\n", $filecontents);

		fwrite($fout, $file_text);
		fclose($fout);
		return true;
	} else {
		return false;
	}
}
OLD:
function add_text_to_file($file, $text) {
	if(file_exists($file) and is_writable($file)) {
		$filecontents = split("\n", file_get_contents($file));
		$fout = fopen($file, "w");
		$new_file_text = "";
		foreach($filecontents as $line) {
			if($line)
				$new_file_text .= rtrim($line) . "\n";
		}
		$new_file_text .= $text . "\n";
		$file_text = str_replace("\n\n", "\n", $new_file_text);
		fwrite($fout, $file_text);
		fclose($fout);
		return true;
	} else {
		return false;
	}
}