Description:
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filter_var($uri, FILTER_VALIDATE_URL) will return false if $uri is a "tel:" 
scheme.

I'm not sure if this is covered by RFC 2396, so if it isn't then this should 
become a feature request instead of a bug.

Test script:
---------------
$uri = 'tel:4166520322';

echo filter_var($uri, FILTER_VALIDATE_URL) ? "ok" : "no";


Expected result:
----------------
ok

Actual result:
--------------
no