Change Text helper default tag from <strong> to <mark>

Supersedes PR #1497
diff --git a/system/helpers/text_helper.php b/system/helpers/text_helper.php
index b2351db..c6d1478 100644
--- a/system/helpers/text_helper.php
+++ b/system/helpers/text_helper.php
@@ -329,25 +329,17 @@
 	 *
 	 * Highlights a phrase within a text string
 	 *
-	 * @param	string	the text string
-	 * @param	string	the phrase you'd like to highlight
-	 * @param	string	the openging tag to precede the phrase with
-	 * @param	string	the closing tag to end the phrase with
+	 * @param	string	$str		the text string
+	 * @param	string	$phrase		the phrase you'd like to highlight
+	 * @param	string	$tag_open	the openging tag to precede the phrase with
+	 * @param	string	$tag_close	the closing tag to end the phrase with
 	 * @return	string
 	 */
-	function highlight_phrase($str, $phrase, $tag_open = '<strong>', $tag_close = '</strong>')
+	function highlight_phrase($str, $phrase, $tag_open = '<mark>', $tag_close = '</mark>')
 	{
-		if ($str === '')
-		{
-			return '';
-		}
-
-		if ($phrase !== '')
-		{
-			return preg_replace('/('.preg_quote($phrase, '/').')/i', $tag_open.'\\1'.$tag_close, $str);
-		}
-
-		return $str;
+		return ($str !== '' && $phrase !== '')
+			? preg_replace('/('.preg_quote($phrase, '/').')/i', $tag_open.'\\1'.$tag_close, $str)
+			: $str;
 	}
 }