php - Extract specific object from JSON array as a comma separated string via PowerShell

I'm trying to write a basic JSON interpreter in PowerShell for a scheduled external port scanner which outputs open ports via JSON, but I'm having a lot of trouble getting it to display the data as a comma separated string. The reason for the comma separated string is so that it can be passed to my documentation tool to make notes of client networks.

My PHP Code (working fine, just for reference):

<?php
$host = $_SERVER['REMOTE_ADDR'];
$ports = array(
    20,
    443,
    49371
    );
echo "[";
foreach ($ports as $port)
{
    $connection = @fsockopen($host, $port, $errno, $errstr, 2);
    if (is_resource($connection))
    {
        echo '{' . '"Port":' . $port . ',' . '"status" : "open"' . "},";
        fclose($connection);
    }
    else
    {
        echo '{' . '"Port":' . $port . ', "status" : "closed"},';
    }
} ?> { "result": "done" } ]
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Sample HTML Output when invoking that PHP script:

[{"Port":20, "status" : "open"},{"Port":443, "status" : "open"},{"Port":49731, "status" : "closed"}, { "result": "done" } ]
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String (as a variable) that I'm trying to achieve in Powershell:

Open Ports = 20, 443
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My Powershell code so far:

$Results = invoke-restmethod -uri "https://mywebsite.com/portscan.php"
$OpenPorts = $Results | Where-Object { $_.status -eq "open"} | ConvertFrom-Json
$Message = "Open Ports: $OpenPorts"
Write-host $Message
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The error I can't figure out how to resolve:

ConvertFrom-Json : Invalid JSON primitive: .
At line:2 char:64
+ ...  = $Results | Where-Object { $_.status -eq "open"} | ConvertFrom-Json
+                                                          ~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [ConvertFrom-Json], ArgumentException
    + FullyQualifiedErrorId : System.ArgumentException,Microsoft.PowerShell.Commands.ConvertFromJsonCommand
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Any help would be sincerely appreciated, I'm not familiar with PowerShell and it's doing my head in!

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