I use documentation with WWWForm: https://docs.unity3d.com/Manual/UnityWebRequest-SendingForm.html

C#:

 void Start() {
     StartCoroutine(Upload());
 }
 
 IEnumerator Upload(){
     yield return Upload1();
     yield return Upload2();
 }

 IEnumerator Upload1() {
     List<IMultipartFormSection> formData = new List<IMultipartFormSection>();
     formData.Add( new MultipartFormDataSection("field1=foo&field2=bar") );
     //formData.Add( new MultipartFormFileSection("my file data", "myfile.txt") );

     UnityWebRequest www = UnityWebRequest.Post(url, formData);
     yield return www.SendWebRequest();
 
     if(www.isNetworkError || www.isHttpError) {
         Debug.Log(www.error);
     }
     else {
         Debug.Log("Form upload complete!");
         Debug.Log( "MULTIPART: " + www.downloadHandler.text );
     }
 }
 
 IEnumerator Upload2() {
     WWWForm form = new WWWForm();
     form.AddField("myField", "myData");
 
     UnityWebRequest www = UnityWebRequest.Post(url, form);
     yield return www.SendWebRequest();
 
     if(www.isNetworkError || www.isHttpError) {
         Debug.Log(www.error);
     }
     else {
         Debug.Log("Form upload complete!");
         Debug.Log( "WWWForm: " + www.downloadHandler.text );
     }
 }

PHP:

 <?php 
 
     echo "POST: ";
     print_r( $_POST );
     var_dump( $_POST );
     
     echo "GET: ";
     print_r( $_GET );
     var_dump( $_GET );
     
 ?>

Response (MULTIPART and WWWForm):

 POST: Array
 (
 )
 array(0) {
 }
 GET: Array
 (
 )
 array(0) {
 }

$_POST is empty... (that's my problem)

Use Unity 2017.3.

I used UnityWebRequest.Send (Unity 5.6) before and it worked for me until it became obsolete.

Thanks.