There might be many instances where you will create a program in PHP that takes a URL as input and does something with the address. This might be a site analysis or an image resize, but whatever the use is, you need to be sure that the URL will work or at least has the same format.
What users tend to leave out of a URL string is the http:// bit at the start. You could validate the URL to force the user to do this, but you will end up annoying a few people. By far the best way of making sure that the URL has the http:// bit at the start is by adding it behind the scenes. The best way to this is to remove the http:// from the start of the string, even if it isn't there and then add it back on.