Why isn't this $_POST working properly?

Howdy, y'all! I was doing something today and seem to be having trouble simply uploading a WWWForm with post info. It's been pretty frustrating and I can't figure out why ha ha. The goal is to send a string from Unity to a PHP file, and that PHP file writes the string to a file called "newFile.txt"

This is happening in Unity:

 IEnumerator UploadMetrics(string inMetrics) {
         WWWForm form = new WWWForm();
         form.AddField("action", "telemetry");
         Debug.Log (inMetrics);
         form.AddField("metric", inMetrics);
         //uploadUrl is a string like "http://mywebsite.com/upload.php"
         WWW w = new WWW(uploadUrl,form);
         yield return w;
         if (w.error != null) {
             Debug.Log(w.error);
         } else {
             Debug.Log ("Successful Upload!");
         }
     }

While this is happening in PHP:

 <?php
     
 if ($_POST) 
 {
     if ( isset ($_POST['action']) ) 
     {
 
        if($_POST['action'] === "telemetry") 
         {
            $thetext = $_POST['metric'];
             $myfile = fopen("newfile.txt", "w") or die("Unable to open file!");
             fwrite($myfile, $thetext);
             fclose($myfile);
         }
 
     }
 
 }
 
 ?>

I've done similar stuff a million times before, generally with interacting with databases via SQL,, but for some reason I'm just not seeing what I'm doing wrong. Any ideas?

Thanks so much!