Short version to output link on a webpage

$url =  "//{$_SERVER['HTTP_HOST']}{$_SERVER['REQUEST_URI']}";

$escaped_url = htmlspecialchars( $url, ENT_QUOTES, 'UTF-8' );
echo '<a href="' . $escaped_url . '">' . $escaped_url . '</a>';

Here are some more details about the issues and edge cases of the //example.com/path/ format

Full version

function url_origin( use_forwarded_host = false )
{
    $ssl      = ( ! empty( $s['HTTPS'] ) && $s['HTTPS'] == 'on' );
    $sp       = strtolower( $s['SERVER_PROTOCOL'] );
    $protocol = substr( $sp, 0, strpos( ssl ) ? 's' : '' );
    $port     = $s['SERVER_PORT'];
    $port     = ( ( ! $ssl && $port=='80' ) || ( $ssl && $port=='443' ) ) ? '' : ':'.$port;
    $host     = ( $use_forwarded_host && isset( $s['HTTP_X_FORWARDED_HOST'] ) ) ? $s['HTTP_X_FORWARDED_HOST'] : ( isset( $s['HTTP_HOST'] ) ? $s['HTTP_HOST'] : null );
    $host     = isset( $host ) ? $host : $s['SERVER_NAME'] . $port;
    return $protocol . '://' . $host;
}

function full_url( use_forwarded_host = false )
{
    return url_origin( use_forwarded_host ) . $s['REQUEST_URI'];
}

$absolute_url = full_url( $_SERVER );
echo $absolute_url;

This is a heavily modified version of http://snipplr.com/view.php?codeview&id=2734 (Which no longer exists)

URL structure:

scheme://username:password@domain:port/path?query_string#fragment_id

The parts in bold will not be included by the function

Notes:

• This function does not include username:password from a full URL or the fragment (hash).
• It will not show the default port 80 for HTTP and port 443 for HTTPS.
• Only tested with http and https schemes.
• The #fragment_id is not sent to the server by the client (browser) and will not be added to the full URL.
• $_GET will only contain foo=bar2 for an URL like /example?foo=bar1&foo=bar2.
• Some CMS's and environments will rewrite $_SERVER['REQUEST_URI'] and return /example?foo=bar2 for an URL like /example?foo=bar1&foo=bar2, use $_SERVER['QUERY_STRING'] in this case.
• Keep in mind that an URI = URL + URN, but due to popular use, URL now means both URI and URL.
• You should remove HTTP_X_FORWARDED_HOST if you do not plan to use proxies or balancers.
• The spec says that the Host header must contain the port number unless it is the default number.

Client (Browser) controlled variables:

• $_SERVER['REQUEST_URI']. Any unsupported characters are encoded by the browser before they are sent.
• $_SERVER['HTTP_HOST'] and is not always available according to comments in the PHP manual: http://php.net/manual/en/reserved.variables.php
• $_SERVER['HTTP_X_FORWARDED_HOST'] gets set by balancers and is not mentioned in the list of $_SERVER variables in the PHP manual.

Server controlled variables:

• $_SERVER['HTTPS']. The client chooses to use this, but the server returns the actual value of either empty or "on".
• $_SERVER['SERVER_PORT']. The server only accepts allowed numbers as ports.
• $_SERVER['SERVER_PROTOCOL']. The server only accepts certain protocols.
• $_SERVER['SERVER_NAME'] . It is set manually in the server configuration and is not available for IPv6 according to kralyk.

Related:

What is the difference between HTTP_HOST and SERVER_NAME in PHP?
Is Port Number Required in HTTP "Host" Header Parameter?
https://stackoverflow.com/a/28049503/175071

If you want just the parts of URL after http://domain.example, try this:

<?php echo $_SERVER['REQUEST_URI']; ?>

If the current URL was http://domain.example/some-slug/some-id, echo will return only /some-slug/some-id.

If you want the full URL, try this:

<?php echo 'http://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI']; ?>