How to change a variable from another script?

Hi, I know that similar questions have been asked, but my question is with a yield www function.

I can load an image from a URL, into a texture, like this:

 public void LoadImage(String img_URL, Texture2D tex=null)
     {    
         www = new WWW(img_URL);
         StartCoroutine(WaitForImageLoad(www, tex));
     }
     
     IEnumerator WaitForImageLoad(WWW www, Texture2D tex=null)
     {        
         yield return www; 
         www.LoadImageIntoTexture(tex);            
     }

But when I try to do the same thing with a String, only the local copy is modified, but not the real variable referenced by the argument theString, in the below code :

 public void LoadString(String URL, String theString=null)
     {    
         www = new WWW(URL);
         StartCoroutine(WaitForStringLoad(www, theString));
     }
     
     IEnumerator WaitForStringLoad(WWW www, String theString=null)
     {    
         yield return www;
         theString = www.text;
     }

What I'm trying to do :

These functions belong to a script called AjaxScript.cs, and I'm trying to initialize the variables of PlayerScript.cs, like this:

 using UnityEngine;
 using System;
 using System.Collections;
 
 public class PlayerScript : MonoBehaviour {
     
     //public variables:
     public AjaxScript AJAX;    
     public Texture2D PlayerPhoto;    
     public String PlayerName="not_defined";
     
     // Use this for initialization
     void Start () {
     
     
             AJAX  = gameObject.GetComponent<AjaxScript>();            
                     
             AJAX.LoadImage("http://image.com/image.jpg",PlayerPhoto); 
             // this works, i.e. PlayerPhoto becomes image.jpg
             
             AJAX.LoadString("http://localhost/my_unity_php_agent.php",this.PlayerName);
             //but this does not work, i.e. PlayerName remains "not_defined"
     }
     
 }

How can I pass PlayerName as a reference and change it direclty in AjaxScript ?

Thanks !