See the PSR URI proposal at https://groups.google.com/forum/?fromgroups=#!topic/php-fig/06BSXGqPRiU.

HierarchicalUriInterface.php

<?php

namespace Psr\Uri;

use Psr\Uri\Exception\InvalidArgumentException;

/**
 * A hierarchical uniform resource identifier (URI) is either an absolute URI
 * whose scheme-specific part begins with a slash ("/"), or a relative URI,
 * that is, a URI that does not specify a scheme.
 *
 * <pre>
 *   foo://user:password@example.com:8042/over/there?name=ferret#teeth
 *   \_/   \___________/ \_________/ \__/\_________/ \_________/ \___/
 *    |          |            |       |       |           |        |
 *    |      user info      host     port     |           |        |
 *    |    \____________________________/     |           |        |
 *    |                  |                    |           |        |
 *    |              authority               path       query   fragment
 *    |  \_________________________________________/
 *    |                       |
 *    |               hierarchical part
 *    |  \___________________________________________________________/
 *    |                                |
 * scheme                    scheme-specific part
 * </pre>
 *
 * Some examples of hierarchical URIs are:
 *
 * - <samp>http://php.net/manual/en/</samp>
 * - <samp>manual/en/language.oop5.interfaces.php</samp>
 * - <samp>../../conferences/</samp>
 * - <samp>file:///~/calendar</samp>
 */
interface HierarchicalUriInterface extends UriInterface
{
    /**
     * Gets the hierarchical part.
     *
     * The hierarchical part is in the format:
     *
     * <pre>
     * [//authority/]path[?query][#fragment]
     * </pre>
     *
     * Implementations MUST return the preceding double slash ("//") if the
     * authority component is present. They MUST NOT return a succeeding
     * question mark ("?") or number sign ("#") that delimits it from a path,
     * query or fragment component respectively.
     *
     * @return string|null Hierarchical part, or `null` if not set.
     */
    public function getHierarchicalPart();

    /**
     * Gets the authority component.
     *
     * The authority component is in the format:
     *
     * <pre>
     * [user-info@]host[:port]
     * </pre>
     *
     * If the port subcomponent has been set as the known default for the
     * scheme component it SHOULD NOT be included.
     *
     * Implementations MUST NOT return the preceding double slash ("//") nor a
     * subsequent slash ("/"), question mark ("?") or number sign ("#") that
     * delimits it from a path, query or fragment component respectively.
     *
     * @return string Authority component, or `null` if not set.
     */
    public function getAuthority();

    /**
     * Gets the user info subcomponent of the authority.
     *
     * Implementations MUST NOT return the succeeding at-sign ("@").
     *
     * @return string|null User info component, or `null` if not set.
     */
    public function getUserInfo();

    /**
     * Sets the user info subcomponent of the authority.
     *
     * A `null` value will remove the existing user info subcomponent.
     *
     * @param string $userInfo User info subcomponent.
     *
     * @return self Reference to the URI.
     *
     * @throws InvalidArgumentException When the user subcomponent is not valid.
     */
    public function setUserInfo($userInfo);

    /**
     * Gets the host subcomponent of the authority.
     *
     * It MUST be treated as case-insensitive.
     *
     * @return string|null Host subcomponent, or `null` if not set.
     */
    public function getHost();

    /**
     * Set the host subcomponent of the authority.
     *
     * The host subcomponent MUST be one of:
     *
     * - A domain name.
     * - An IPv4 address.
     * - An IPv6 address.
     *
     * The host MUST be a string, or an object that implements the
     * `__toString()` method.
     *
     * A `null` value will remove the existing host subcomponent.
     *
     * @param string $host Host subcomponent.
     *
     * @return self Reference to the URL.
     *
     * @throws InvalidArgumentException When the host subcomponent is not valid.
     */
    public function setHost($host);

    /**
     * Gets the port subcomponent of the authority.
     *
     * If the port has not been explicitly set using `setPort()` this MAY
     * return the default port for the scheme.
     *
     * @return int|null Port subcomponent, or `null` if not set.
     */
    public function getPort();

    /**
     * Sets the port subcomponent of the authority.
     *
     * The port subcomponent MUST be a non-negative integer.
     *
     * A `null` value will remove the existing port subcomponent.
     *
     * @param int $port Port subcomponent.
     *
     * @return self Reference to the URI.
     *
     * @throws InvalidArgumentException When the port subcomponent is not valid.
     */
    public function setPort($port);
}

OpaqueUriInterface.php

<?php

namespace Psr\Uri;

/**
 * An opaque uniform resource identifier (URI) is an absolute URI whose
 * scheme-specific part does not begin with a slash character ("/").
 *
 * <pre>
 *   urn:example:animal?name=ferret#teeth
 *   \_/ \____________/ \_________/ \___/
 *    |         |            |        |
 *    |        path        query   fragment
 *    |  \____________/
 *    |         |
 *    | hierarchical part
 *    |  \______________________________/
 *    |                 |
 * scheme     scheme-specific part
 * </pre>
 *
 * Some examples of opaque URIs are:
 *
 * - <samp>mailto:example@example.com</samp>
 * - <samp>news:comp.lang.php</samp>
 * - <samp>urn:isbn:096139210x</samp>
 */
interface OpaqueUriInterface extends UriInterface
{
    /**
     * Gets the hierarchical part.
     *
     * The hierarchical part for opaque URIs is the path.
     *
     * @return string|null Hierarchical part, or `null` if not set.
     */
    public function getHierarchicalPart();
}

UriInterface.php

<?php

namespace Psr\Uri;

use Psr\Uri\Exception\InvalidArgumentException;

/**
 * Represents a uniform resource identifier (URI), which identifies an abstract
 * or physical resource.
 *
 * <pre>
 *                 scheme-specific part
 *        __________________|___________________
 *       /                                      \
 *   foo://example.com:8080/bar?name=ferret#teeth
 *   \_/   \__________________/ \_________/ \___/
 *    |             |                |        |
 * scheme   hierarchical part      query   fragment
 *    |   __________|_________   ____|____   _|_
 *   / \ /                    \ /         \ /   \
 *   urn:example:example:animal?name=ferret#teeth
 *       \______________________________________/
 *                          |
 *                 scheme-specific part
 * </pre>
 *
 * This interface is not to be implemented directly, instead implement
 * `OpaqueUriInterface` or `HierarchicalUriInterface` as appropriate.
 */
interface UriInterface
{
    /**
     * Returns the URI as a string.
     *
     * @return string URI as a string.
     */
    public function __toString();

    /**
     * Whether this URI is absolute or not.
     *
     * A URI is absolute if, and only if, it has a scheme component.
     *
     * @return bool `true` if the URI is absolute, otherwise `false`.
     */
    public function isAbsolute();

    /**
     * Gets the scheme component.
     *
     * It MUST be treated as case-insensitive.
     *
     * Implementations MUST NOT return the succeeding colon (":").
     *
     * @return string|null Scheme component, or `null` if not set.
     */
    public function getScheme();

    /**
     * Sets the scheme component
     *
     * A `null` value will remove the existing scheme component.
     *
     * @param string $scheme Scheme component.
     *
     * @return self Reference to the URI.
     *
     * @throws InvalidArgumentException When the scheme component is not valid.
     */
    public function setScheme($scheme);

    /**
     * Gets the scheme-specific part.
     *
     * Implementations MUST NOT return the preceding colon (":").
     *
     * @return string|null Scheme-specific part, or `null` if not set.
     */
    public function getSchemeSpecificPart();

    /**
     * Gets the hierarchical part.
     *
     * @return string|null Hierarchical part, or `null` if not set.
     */
    public function getHierarchicalPart();

    /**
     * Gets the path.
     *
     * Implementations MUST return an empty string (as opposed to `null`) when
     * no path is set.
     *
     * @return string Path.
     */
    public function getPath();

    /**
     * Sets the path component.
     *
     * The query MUST be a string, or an object that implements the
     * `__toString()` method. An implementation MAY also accept an array.
     *
     * Implementations MAY treat a `null` value as an empty string.
     *
     * @param string $path Path component.
     *
     * @return self Reference to the URI.
     *
     * @throws InvalidArgumentException When the path component is not valid.
     */
    public function setPath($path);

    /**
     * Gets the query component.
     *
     * Implementations MUST NOT return the preceding question mark ("?") nor a
     * succeeding number sign ("#").
     *
     * @return string|null Query component, or `null` if not set.
     */
    public function getQuery();

    /**
     * Sets the query component.
     *
     * The query MUST be a string, or an object that implements the
     * `__toString()` method. An implementation MAY also accept an associative
     * array.
     *
     * A `null` value will remove the existing query component.
     *
     * @param mixed $query Query component.
     *
     * @return self Reference to the URI.
     *
     * @throws InvalidArgumentException When the query component is not valid.
     */
    public function setQuery($query);

    /**
     * Gets the fragment component.
     *
     * Implementations MUST NOT return the preceding number sign ("#").
     *
     * @return string|null Fragment component, or `null` if not set.
     */
    public function getFragment();

    /**
     * Sets the fragment component.
     *
     * The fragment component MUST be a string, or an object that implements the
     * `__toString()` method.
     *
     * A `null` value will remove the existing fragment component.
     *
     * @param string $fragment Fragment component.
     *
     * @return self Reference to the URI.
     *
     * @throws InvalidArgumentException When the fragment component is not valid.
     */
    public function setFragment($fragment);
}