[CORS Proxy] Support chunked encoding when running in Apache/Nginx/etc (

#2114)

Adds support for `transfer-encoding: chunked` on Apache, Nginx, and
other web servers.

#2077 added support for `transfer-encoding: chunked` in a way that works
on a local PHP dev server. The proxy manually chunks the output bytes
and outputs each chunk's header, separator, body, and trailer.

However, it doesn't work on playground.wordpress.net because the web
server there handles the chunked encoding on its own. The bytes echoed
by cors-proxy.php are treated as body bytes, which messes up the
response body.

This PR restricts the manual chunking to a local CLI dev server.

## Testing instructions

Run cors-proxy.php in Apache or so and confirm that requesting resources
served with chunked encoding works with this patch but not without it.
One such URL is https://adamadam.blog/feed/.

packages/playground/php-cors-proxy/cors-proxy.php

@@ -97,16 +97,33 @@
 };
 
 function send_response_chunk($data) {
-    global $is_chunked_response;
-    if ($is_chunked_response) {
+    if (should_send_as_chunked_response()) {
+        // We need to manually chunk the response when running in the PHP
+        // built-in server. It won't handle that for us.
         echo sprintf("%s\r\n%s\r\n", dechex(strlen($data)), $data);
     } else {
+        // When running behing an Apache or Nginx or another webserver,
+        // it will handle the chunking for us. Manually sending the chunk
+        // header, \r\n separator, body, and \r\n trailer isn't just
+        // unnecessary, but it would actually include those bytes in the
+        // response body.
         echo $data;
     }
     @ob_flush();
     @flush();
 }
 
+/**
+ * We need to manually chunk the response when running the PHP
+ * dev server AND the transfer-encoding header is set to chunked.
+ * 
+ * Apache, Nginx, etc. will handle the chunking for us.
+ */
+function should_send_as_chunked_response() {
+    global $is_chunked_response;
+    return $is_chunked_response && php_sapi_name() === 'cli-server';
+}
+
 // Pin the hostname resolution to an IP we've resolved earlier
 curl_setopt($ch, CURLOPT_RESOLVE, [
     "$host:80:$resolvedIp",
@@ -255,7 +272,9 @@ function(
 // Close cURL session
 curl_close($ch);
 
-// Only send chunked transfer encoding footer if we're using chunked encoding
-if ($is_chunked_response) {
+// Only send chunked transfer encoding footer if we're using chunked encoding.
+// We need to manually send the footer when running in the PHP built-in server
+// because, unlike apache or nginx, it won't handle that for us.
+if (should_send_as_chunked_response()) {
     echo "0\r\n\r\n";
 }