Try this:

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

Learn more about the $_SERVER predefined variable.

If you plan on using https, you can use this:

function url(){
  return sprintf(
    "%s://%s%s",
    isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] != 'off' ? 'https' : 'http',
    $_SERVER['SERVER_NAME'],
    $_SERVER['REQUEST_URI']
  );
}

echo url();
#=> http://127.0.0.1/foo

Per this answer, please make sure to configure your Apache properly so you can safely depend on SERVER_NAME.

<VirtualHost *>
    ServerName example.com
    UseCanonicalName on
</VirtualHost>

NOTE: If you’re depending on the HTTP_HOST key (which contains user input), you still have to make some cleanup, remove spaces, commas, carriage return, etc. Anything that is not a valid character for a domain. Check the PHP builtin parse_url function for an example.