I have $_SERVER[\'HTTP_REFERER\'] — pretend it is http://example.com/i/like/turtles.html. What would I need to do to get just the http://example.com
In this example, the best solution would be to use PHP's parse_url method. This splits up the URL into an associative array. You would then build your final value by combining the scheme with the host:
if ( $parts = parse_url( "http://example.com/i/like/turtles.html" ) ) {
echo $parts[ "scheme" ] . "://" . $parts[ "host" ];
}