<?
$ret = 'I gave my Word to you Word.';
$pattern = '/\bWord\b/i';
$ret = preg_replace($pattern,"Heart",$ret);
var_dump($ret);
?>
string(29) "I gave my Heart to you Heart."
Or if this is what suits you
You could just use str_replace:
$str = str_replace(array('<tag>', '</tag>'), array('<newtag>', '</newtag>', $str);