using UnityEngine;
using System.Collections;
using UnityEngine.UI;

public class UIManager : MonoBehaviour
{
    public InputField output;

    public void SaveStrOnClick()
    {
        StartCoroutine("SaveStr");
    }

    IEnumerator SaveStr()
    {
        WWWForm form = new WWWForm();
        form.AddField("str", "Hello");
        WWW www = new WWW("http://dev3dapps.freeoda.com/unity/Polyglot/database.php", form);
        yield return www;
        output.text = www.text;
    }
}

<?php

if (!empty($_POST["str"]))
{
    $str = $_POST["str"];
    echo "Ok, we get: ".$str;
}
else
{
   echo "Error: cannot get str";
}