unable to receive http post using php

Question

I have a web site with php code that store values received from an http post in a database.

Is made by three files

add.php (to receive and store)

<?php
    include("connect.php");

    $link=Connection();

    $temp=$_POST["temp"];
    $lat =$_POST["lat"];
    $lng =$_POST["lng"];

    $query = "INSERT INTO `temptrac_temp` (`ID`, `temp`, `lat`, `lng`, `timestamp`) VALUES (NULL,'".$temp."', '".$lat."', '".$lng."', CURRENT_TIMESTAMP)";


    mysqli_query($link, $query);


    //mysqli_free_result($query);

    mysqli_close($link);

    header("Location: index.php");
?>

connect.php (to access to mysql server)

<?php

    function Connection(){


        $mysqli = new mysqli("localhost", "temptrac_temp", "temptrac_temp2846", "temptrac_temp");

        /* check connection */
        if ($mysqli->connect_errno) {
            printf("Connect failed: %s\n", $mysqli->connect_error);
            exit();
        }

        return $connection;
    }
?>

index.php (main page that list the data from the database)

<?php

    include("connect.php");     

    $link=Connection();

    $result=mysqli_query($link, "SELECT * FROM `temptrac_temp` ORDER BY `timestamp` DESC");
?>



<p>Temperature Arduino checker</p>

<p>The temperature is:</p>  

<table class="normal" border="2" cellspacing="1" cellpadding="1">

    <tr>
        <td>&nbsp;Temperature&nbsp;</td>
        <td>&nbsp;Latitude&nbsp;</td>
        <td>&nbsp;Longitude&nbsp;</td>
        <td>&nbsp;Timestamp&nbsp;</td>

    </tr>

    <?php 
        if($result!==FALSE){
            printf("porcozio");
            while($row = mysqli_fetch_array($result)) {
                printf("<tr><td> &nbsp;%s </td><td> &nbsp;%s&nbsp; </td><td> &nbsp;%s&nbsp; </td><td> &nbsp;%s&nbsp; </td></tr>", 
                    $row["temp"], $row["lat"], $row["lng"], $row["timestamp"]);

            }
            mysqli_free_result($result);
            mysqli_close();
        }else{

            printf("there is an error");

        }
    ?>

</table>

I do not receive any error so seem like that can connect to mysql server successfully and retrieve 0 rows from the table in the database.

I start to have problem when I modify the code because some function were deprecated and I move to the newer php version.

I remember that my database is already populated and I try different php version in my domine.

here my table

ID (int) | temp (temperature/float) | lat (latitude/float) | lng (longitude/float) | timestamp (timestamp)|

I make the http post by using this site https://www.hurl.it/

Unfortunately I'm not an expert with php and I wonder if there is a good debugger to understand what is really happening.

thanks in advice.

bye!