为什么输入if语句时我的数组值会消失？

I'm bringing multiple queried values to my front end and doing so with an array. When I echo my value of $hold['POAmount']; before my if statement it returns as expected 253{"Sum":10,"POAmount":-10} although don't confuse this with my desired output. But after entering my if statement the response back to the page is {"Sum":10,"POAmount":-10}. I thought maybe using fetch_assoc() over fetch_array() would do something but this does nothing. Heres what I got:

<?php
include '../../inc/dbinfo.inc';

ini_set("log_errors", 1);
ini_set("error_log", "/tmp/php-error.log");
error_log( "#################################################     FT-SUM-INVOICES.PHP    #################################################" );

$inv = $_POST['inv'];
$status = $_POST['stat'];

//Get PO # From invoice id
$getPO = "SELECT VendorPOID1, VdrInvoiceAmount FROM tblVendorInvoices WHERE VENDORINVOICEID =?";
$exgetPO = $conn->prepare($getPO);
$exgetPO->bind_param("i", $inv);
$exgetPO->execute();
$obj = $exgetPO->get_result();
$hold = $obj->fetch_assoc();
$po = $hold['VendorPOID1'];
$currentInAmount = $hold['VdrInvoiceAmount'];

$cn = 3; //paid status

$getPOAmount = "SELECT POAmount FROM tblVendorPOs WHERE VENDORPOID = ?";
$poAmount = $conn->prepare($getPOAmount);
$poAmount->bind_param("i",$po);
$poAmount->execute();
$obj3 = $poAmount->get_result();
$hold = $obj3->fetch_assoc(); //Total PO Amount


if($status == 3){
  echo json_encode($hold['POAmount']);
  //Get Sum of invoices on po + current invoice
  $qr = "SELECT SUM(VdrInvoiceAmount) AS Sum FROM tblVendorInvoices WHERE VENDORPOID1 = ? AND VdrInvoiceStatusID = ?";
  $getSum = $conn->prepare($qr);
  $getSum->bind_param("ii", $po, $cn);
  $getSum->execute();
  $obj2 = $getSum->get_result();
  $hold = $obj2->fetch_assoc();

  $hold['Sum'] += $currentInAmount;

  $hold['POAmount'] -= $hold['Sum'];  //remaining balance

  if($hold['Sum'] == NULL){
    $hold['Sum'] = 0;
  }

  echo json_encode($hold);
}else{
 //Get Sum of invoices on po
 $qr = "SELECT SUM(VdrInvoiceAmount) AS Sum FROM tblVendorInvoices WHERE VENDORPOID1 = ? AND VdrInvoiceStatusID = ?";
  $getSum = $conn->prepare($qr);
  $getSum->bind_param("ii", $po, $cn);
  $getSum->execute();
  $obj2 = $getSum->get_result();
  $hold = $obj2->fetch_assoc();
  if($hold['Sum'] == NULL){
    $hold['Sum'] = 0;
  }

  $hold['POAmount'] -= $hold['Sum'];

  echo json_encode($hold);
}

?>

My if statement I am describing above is if($status == 3){ I am not receiving any errors from the web page or my PHP error log.

Summary of my problem:

$hold['POAmount'] is considered null when I subtract sum from it. So it is coming up as -$hold['Sum']. Whereas it shouldn't be null to begin with. It is becoming null upon entering my if statement.

Current Output: {"Sum":9,"POAmount":-9} Desired Output: {"Sum":9,"POAmount":243}

This should work (haven't tested it). What I've changed is, I've assigned the result of the SELECT query within the if statement to a different variable named $hold2, so that the $hold['POAmount'] value in $hold isn't overwritten.

And then I've merged the array in the $hold variable (which holds the result from the SELECT query right before the if statement) with the array in $hold2 (which holds the result from the SELECT query inside the if statement), which produces an array with both the POAmount and Sum keys in it.

Let me know if this isn't working and what errors you're getting, I'd be happy to help. :)

<?php
include '../../inc/dbinfo.inc';

ini_set("log_errors", 1);
ini_set("error_log", "/tmp/php-error.log");
error_log( "#################################################     FT-SUM-INVOICES.PHP    #################################################" );

$inv = $_POST['inv'];
$status = $_POST['stat'];

//Get PO # From invoice id
$getPO = "SELECT VendorPOID1, VdrInvoiceAmount FROM tblVendorInvoices WHERE VENDORINVOICEID =?";
$exgetPO = $conn->prepare($getPO);
$exgetPO->bind_param("i", $inv);
$exgetPO->execute();
$obj = $exgetPO->get_result();
$hold = $obj->fetch_assoc();
$po = $hold['VendorPOID1'];
$currentInAmount = $hold['VdrInvoiceAmount'];

$cn = 3; //paid status

$getPOAmount = "SELECT POAmount FROM tblVendorPOs WHERE VENDORPOID = ?";
$poAmount = $conn->prepare($getPOAmount);
$poAmount->bind_param("i",$po);
$poAmount->execute();
$obj3 = $poAmount->get_result();
$hold = $obj3->fetch_assoc(); //Total PO Amount


if($status == 3){
  echo json_encode($hold['POAmount']);
  //Get Sum of invoices on po + current invoice
  $qr = "SELECT SUM(VdrInvoiceAmount) AS Sum FROM tblVendorInvoices WHERE VENDORPOID1 = ? AND VdrInvoiceStatusID = ?";
  $getSum = $conn->prepare($qr);
  $getSum->bind_param("ii", $po, $cn);
  $getSum->execute();
  $obj2 = $getSum->get_result();
  $hold2 = $obj2->fetch_assoc();

  $hold = array_merge($hold, $hold2);

  $hold['Sum'] += $currentInAmount;

  $hold['POAmount'] -= $hold['Sum'];  //remaining balance

  if($hold['Sum'] == NULL){
    $hold['Sum'] = 0;
  }

  echo json_encode($hold);
}else{
 //Get Sum of invoices on po
 $qr = "SELECT SUM(VdrInvoiceAmount) AS Sum FROM tblVendorInvoices WHERE VENDORPOID1 = ? AND VdrInvoiceStatusID = ?";
  $getSum = $conn->prepare($qr);
  $getSum->bind_param("ii", $po, $cn);
  $getSum->execute();
  $obj2 = $getSum->get_result();
  $hold2 = $obj2->fetch_assoc();

  $hold = array_merge($hold, $hold2);

  if($hold['Sum'] == NULL){
    $hold['Sum'] = 0;
  }

  $hold['POAmount'] -= $hold['Sum'];

  echo json_encode($hold);
}

?>