Zoho Get an Invoice - php

Question

I am trying to integrate zoho with my web app. I am making an api call to get a particular invoice. The call as per documentation is
'https://books.zoho.com/api/v3/invoices/INV_ID?organization_id=XXXXXXXX'
This works fine as well. But when I set a variable for INVOICE ID, it returns all the invoices. (200 INVOICES) The call is below.
'https://books.zoho.com/api/v3/invoices/' .$inv_id. '?organization_id=XXXXXXXX';
where I am making the mistake? The method is below.
$zid = '';
if(isset($_POST['_zid'])){
$zid = $_POST['_zid'];
}
get_invoice($access_token, $zid);
function get_invoice($access_token, $zid){
$invid = $zid;
$service_url = $GLOBALS['service_url'] . 'https://books.zoho.com/api/v3/invoices/' .$invid. '?organization_id=XXXXXXXXX';
$ch = curl_init();
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
curl_setopt($ch, CURLOPT_URL, $service_url);
curl_setopt($ch, CURLOPT_HTTPHEADER, array(
'Authorization: Zoho-oauthtoken '. $access_token,
'Content-Type: "application/json"'));
$res = curl_exec($ch);
curl_close($ch);
$dec_res = array();
$dec_res = json_decode($res, true);
print_r($dec_res);
}

Answer 1

Everything is fine except a small thing. I didn't think of Array Declaration. The working call is straightly given an integer as the invoice_id. when I pass the variable declared as a string. so although the digits it is counted as a string which makes the call invalid.
//THIS WILL DO THE TRICK
$zid = 0;
if(isset($_POST['_zid'])){
$zid = $_POST['_zid'];
}