$ _POST in php does not return any data

I have written the following php called fetchdetails.php

<?php
$db ="fb";
$con = mysql_connect("host","user","paswd") or die ("hi ".mysql_error());
mysql_select_db($db) or die("helo".mysql_error());

$id = $_POST["id"];
echo $id;
$id = (string)$id;
echo $id."hi";

when i am calling it using www.anyaddress.com/fetchdetails.php?id=1722315 but echo $id is not showing anything. Need help on this asap. Actually i did this the same thing earlier in some other code and it was working fine.

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If want to get the values from a URL query such as the id part from

http://example.com?id=42

You should always use the $_GET super-global array as in:

$id = $_GET['id'];
echo $id; // 42

On the otherhand, you should use $_POST to get any data submitted via an HTML form. You use $_POST["id"]; only when you want to get queries submitted via a form

Whatever the case maybe, you should not trust any data passed by either POST/GET. Therefore, before you put it in you mysql query, you should sanitize/escape it.

Like this: $id = mysql_real_escape_string($_GET['id']),

Note. Every function that starts with mysql_ is depredicated is unsafe to use, so you must learn about PDO or MySQLi