Description:
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HTTP_URL_STRIP_SCHEME and HTTP_URL_STRIP_HOST don't exist but would be nice to have.

HTTP_URL_STRIP_SCHEME is useful to ensure local links stay at the secure or non-secure site.
HTTP_URL_STRIP_HOST is useful when a site has multiple names and one wants (user-generated) links to stay at the same name.

Test script:
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<?php
$s = 'http://example.com/php';
$s2 = http_build_url($s, array(), HTTP_URL_STRIP_SCHEME);
echo $s . "<br>\n";

Expected result:
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//example.com/php