If it's certain defined words you wish to emphasise then you can do it using the same method as above.
e.g. if you wanted to turn any reference to your name into a simple, clickable email link or simply style it…
PHP Code:
$text = ereg_replace("Your Name","<a href=\"mailto:yourname@yourdomain.com\">Your Name</a>",$text);
or
PHP Code:
$text = ereg_replace("Your Name","<strong>Your Name</strong>",$text);
If you want to allow users/clients to input their own data and have styling applied as and when they want then that's when things get a tad more complex - and I bow out, having hit my ceiling.
You might want to check out the PHP sections of Hotscripts.com to see what's available.
You may well find precisely the facility you're after.
http://www.hotscripts.com/PHP/Tips_and_Tutorials/