Well, regex is probably the best answer. Keep in mind that the more idiot proof you try to make this regex, the more likely it is that someone will become a better idiot and insert something invalid that slips though. That said... :-)
print eregi("^http[s]?://[[:alnum:][:punct:]]+",$url);
should get you started.
It's not perfect, it needs to exclude - from the hostname, and check for a slash on the end etc...
