PHP - 如何可用，如何重定向到https？

I know how to redirect to https with PHP, but does anyone know how to redirect only if the site is requested via HTTP and HTTPS is available on the server?

I don't know of a way to check a server's available protocols via php. Also, if there is one, you get lost when you want to check and redirect to a remote server. So in order to check wether your destination server is capable of handling https request you need to query it. Here is an example with php-curl:

<?php
/**
 * Check wether a destination is reachable.
 * 
 * @param string $uri uri to check
 * 
 * @return bool
 */
function checkAvailability($uri) {
    $handle = curl_init($uri);
    curl_setopt_array($handle, [
        CURLOPT_RETURNTRANSFER  => 1,
        CURLOPT_USERAGENT       => 'Mozilla/5.0 (Windows NT 6.1; WOW64; rv:40.0) Gecko/20100101 Firefox/40.1'
        ]);
    $r = curl_exec($handle);
    $responseCode = (int)curl_getinfo($handle, CURLINFO_HTTP_CODE);
    curl_close($handle);
    return $responseCode > 199 && $responseCode < 400;
}

//we test by checking the webpages of two of my local newspapers, l-iz.de will succeed, lvz.de will not
var_dump(checkAvailability('https://www.l-iz.de'));
var_dump(checkAvailability('https://lvz.de'));

The method checkAvailability simple tests if a curling url returns a success HTTP-Code between 200 and 399. This is not as accurate as it could be, but shall be sufficient for this use case. So if you call this method with a https url you get your desired info wether a webserver accepts https traffic

Please have a look at this..

<?php
if (isset($_SERVER['HTTPS']) && ($_SERVER['HTTPS'] == 'on' || $_SERVER['HTTPS'] == 1) || isset($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https') {
    // Enter 'https' URL here...(Eg: https://google.com)
} else {
    // Enter 'http' URL here... (Eg: http://google.com)
}
?>