converting string to URL in PHP

converting string to URL in PHP

I have a string in php and I want to convert it into a URL,Basically I am
taking input from the user so I am not sure either he write
http://www.google.com or just www.google.com or maybe just google.com, In
either cases I have to show him the URL link in an anchor tag. I am
looking for any PHP function that can do this, Otherwise I have to
manually search for http and other If else condition will surely gonna
waste my time,If there is no function I would be more than happy to write
one and make it open to the community. EDIT: Another thing is the best way
to write HTML in PHP, I mean is it good if we use echo' long HTML Forms'
etc any solution Thanks