Function problem... urlencode()?

[Anon]

I have this function:
function boldWord($string, $keyWord) {
$kWordArray = split("[ +.-|&%]", $keyWord);
$clr = "#3399ff";

	while(list(, $kWord) = each($kWordArray)) {
		if($kWord != "") {	
			$kWord = rawurlencode($kWord);

			$string = str_replace($kWord, "<b><font color=\"$clr\">$kWord</font></b>", $string);
			$string = str_replace(strtoupper($kWord), "<b><font color=\"$clr\">" . strtoupper($kWord) . "</font></b>", $string);		
			$string = str_replace(strtolower($kWord), "<b><font color=\"$clr\">" . strtolower($kWord) . "</font></b>", $string);
			$string = str_replace(ucfirst($kWord), "<b><font color=\"$clr\">" . ucfirst($kWord) . "</font></b>", $string);
		}
	}

	return $string;
}

The function is working fine. But it doesn't operate correctly whenever it is used to process a string containing characters such as apostophies. I.e. email addresses. What is wrong???