PHP Multi curl with different ports

I am working with an IOT system and trying to improve our data ingestion. Currently we are iterating over an array of URLs with different ports and sending one curl request at a time. I would like to use multicurl to get around this bottleneck but I am getting only getting errors as a response. Here's my code (IP addresses redacted)

$url = 'http://XXX.XXX.XX.XXX/getvar.csv';
$ports = [8101,8102,8103,8104];

foreach ($ports as $port) {
        $worker = curl_init($curl_url);
        curl_setopt_array($worker, [
            CURLOPT_RETURNTRANSFER => true,
            CURLOPT_POST => true,
            CURLOPT_HEADER => 0,
            CURLOPT_TIMEOUT => 8,
            CURLOPT_POSTFIELDS => $content,
            CURLOPT_PORT => $port
        ]);

        curl_multi_add_handle($mh, $worker);
    }
$i=0;
for (; ;) {
    $still_running = null;
    echo "<p>exec multi curl $i</p>";
    do {
        echo "<p>curl multi exec</p>";
        $err = curl_multi_exec($mh, $still_running);
    } while ($err === CURLM_CALL_MULTI_PERFORM);
    if ($err !== CURLM_OK) {
        $description = curl_strerror(curl_errno($mh));
        echo "<p>Error description: $description</p>";
        // handle curl multi error?
    }
    if ($still_running < 1) {
        echo "<p>curl multi finished</p>";
        // all downloads completed
        break;
    }
    // some haven't finished downloading, sleep until more data arrives:
    curl_multi_select($mh, 8);
    $i++;
}

$curl_time_stop = microtime(true);
$curl_duration += $curl_time_stop - $curl_time_start;
echo "CURL finished";
$results = [];
while (false !== ($info = curl_multi_info_read($mh))) {
    if ($info["result"] !== CURLE_OK) {
        echo "Error ".curl_strerror($info["result"]);
        // handle download error?
    }
    $results[curl_getinfo($info["handle"], CURLINFO_EFFECTIVE_URL)] = 
    curl_multi_getcontent($info["handle"]);
    curl_multi_remove_handle($mh, $info["handle"]);
    curl_close($info["handle"]);
}
curl_multi_close($mh);
var_dump($results);

Here's the errors/response I am getting

Error Server returned nothing (no headers, no data)
results int(1) int(52) resource(29) of type (curl)

From what I can tell error code 52 corresponds to CURLE_GOT_NOTHING. Is there something I am missing? Or is there a difference in the protocols used by multi-curl vs curl?

[Edit: Add verbose output] Here's the result when I use CURLOPT_VERBOSE=>1

*   Trying 1XX.XX.XX.XX:8101...

* Found bundle for host 1XX.XX.XX.XX: 0x2625a00 [serially]
* Server doesn't support multiplex (yet)
* Hostname 1XX.XX.XX.XX was found in DNS cache
*   Trying 1XX.XX.XX.XX:8101...

*** Connected to 1XX.XX.XX.XX (1XX.XX.XX.XX) port 8101 (#0)
> POST /getvar.csv? HTTP/1.1
Host: 1XX.XX.XX.XX:8101
Accept: */*
Content-Length: 248
Content-Type: application/x-www-form-urlencoded**

* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< Content-Type: text/csv; charset="utf-8"
< Server: embedded HTTPD
< Expires: 1 JAN 2013 00:00:00 GMT
< Last-Modified: 4 MAR 2021 12:10:09 GMT
< Cache-Control: no-cache
< Transfer-Encoding: chunked

* Connection #0 to host 1XX.XX.XX.XX left intact
* Empty reply from server
* Closing connection 25

Solution

I found an external issue to my code, but am posting the final, working code here for future reference. Multi-curl does not like iterating over the port number so I had to bake the port into the curl URL and iterate over an array of URLs. The $content is not required for the multicurl to work, but it is to get data from the URLs I'm calling.

<?php


$lists=[
    "LocalBoardTemp"
];
$url = 'http://XXX.XXX.XXX.XXX:';

$ports = [8101,8102,8103,8104,8105,8106,8107,8108,8109];

foreach ($ports as $port) {
    $curl_urls[] = $url.$port.'/getvar.csv?';
}
$content = '';
foreach ($lists as $list1) {
    $content = $content . "&name=" . $list1;
}

$mh = curl_multi_init();
$fp = fopen(dirname(__FILE__).'/curl_errorlog.txt', 'w');
$i=0;
foreach ($curl_urls as $url) {
    $worker = curl_init($url);
    curl_setopt_array($worker, [
        CURLOPT_RETURNTRANSFER => true,
        CURLOPT_POST => true,
        CURLOPT_HEADER => 0,
        CURLOPT_TIMEOUT => 8,
        CURLOPT_POSTFIELDS => $content,
        CURLOPT_VERBOSE=>1,
        CURLOPT_STDERR => $fp
//        ,
//        CURLOPT_PORT => $port
    ]);
    curl_multi_add_handle($mh, $worker);
}
for (; ;) {
    $still_running = null;
    echo "<p>exec multi curl $i</p>";
    do {
        echo "<p>curl multi exec</p>";
        $err = curl_multi_exec($mh, $still_running);
    } while ($err === CURLM_CALL_MULTI_PERFORM);
    if ($err !== CURLM_OK) {
        $description = curl_strerror(curl_errno($mh));
        echo "<p>Error description: $description</p>";
        // handle curl multi error?
    }
    if ($still_running < 1) {
        echo "<p>curl multi finished</p>";
        // all downloads completed
        break;
    }
    // some haven't finished downloading, sleep until more data arrives:
    curl_multi_select($mh, 8);
    $i++;
}

echo "CURL finished";
$results = [];
$j = 0;
while (false !== ($info = curl_multi_info_read($mh))) {

    if ($info["result"] !== CURLE_OK) {
        echo "Error ".curl_strerror($info["result"]);
        // handle download error?
    }
    echo "<p>results ";
    foreach ($info as $x) {
        var_dump($x);
    }
    echo "</p>";
    $results[curl_getinfo($info["handle"], CURLINFO_EFFECTIVE_URL)] = curl_multi_getcontent($info["handle"]);
    echo "<p>result index $j". curl_multi_getcontent($info["handle"])."</p>";
    curl_multi_remove_handle($mh, $info["handle"]);
    curl_close($info["handle"]);
    $j++;
}
curl_multi_close($mh);
echo "<p>All results dump: ";
var_dump($results);
echo "</p>";
fclose($fp);
echo "finished";

This has been cobbled together from several tutorials and stack overflow answers, so I'm sure there is a more efficient way of doing this, but it works.