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Convert NGINX rewrite rule to PHP
if ($http_host !~ "([a-zA-Z0-9.-]+).example.com(.+)") {
    rewrite  ^ http://example.com/browse.php?u=http://$1$2? permanent;
}

<?php
function unparse_url($parsed_url) {
    $scheme   = isset($parsed_url['scheme']) ? $parsed_url['scheme'] . '://' : '';
    $host     = isset($parsed_url['host']) ? $parsed_url['host'] : '';
    $port     = isset($parsed_url['port']) ? ':' . $parsed_url['port'] : '';
    $path     = isset($parsed_url['path']) ? $parsed_url['path'] : '';
    $query    = isset($parsed_url['query']) ? '?' . $parsed_url['query'] : '';
    $fragment = isset($parsed_url['fragment']) ? '#' . $parsed_url['fragment'] : '';
}
if(preg_match('/^([a-zA-Z0-9.-]+).example.com/(.+)$/', $_SERVER["SERVER_NAME"])){
    unparse_url(parse_url($_SERVER["SERVER_NAME"]));
    header('Location: $schemeexample.com/browse.php?u=$scheme$host$port$path$query$fragment');
}
else {
?>
<html>
**HTML CONTENT HERE**

if(preg_match('#([w.-]+).example.com(.+)#', $_SERVER['SERVER_NAME'].$_SERVER['REQUEST_URI'], $match)) {
    header('Location: http://example.com/browse.php?u=http://'.$match[1].$match[2]);
    die;
}