Universal Resource Identifier (URI)

Universal Resource Identifier is a unique sequence of characters that identifies a logical or physical resource used by web technologies.

URI are strings, with a special format, also called scheme, which allows identify uniquely a resource, without providing its location.

For example, a ISBN is a unique universal identifier for a book, yet it doesn’t mention where to find such book.

URI is closely related to URL, which provides the location of the desired resource.

Documentation

See also https://en.wikipedia.org/wiki/List_of_URI_schemes

Related : Universal Resource Locator (URL), File, file_get_contents(), URI Class, URI Extension