Using GET, POST and GLOBALS - php

Question

I'm trying to sort table of data by it's columns(as hyperlinks), but also to navigate using < or > buttons.
<?php
...
if (isset($_POST["next"]) && isset($_POST["start"]))
{
//force start to be an integer and add 5 to it
$start = (int)$_POST["start"] + 5;
}
elseif (isset($_POST["prev"]) && isset($_POST["start"]))
{
//force start to be an integer and remove 5 from it
$start = (int)$_POST["start"] - 5;
}
else
{
$start = 0;
}
// if one of hyperlinks in table heading was clicked
if (isset($_GET["sort"]))
{
$orderByFields = ["TerritoryDescription", "RegionDescription"];
$GLOBALS['sort'] = $_GET["sort"];
...
SQL here.
}
else
{
...
}
//problem here
// if next button was pressed
if (isset($_POST['next']))
{
echo 'next';
if (isset($_GET['sort']))
{
echo 'sort';
//undefined
//i'm trying to retrive value here.
echo $GLOBALS['sort'];
}
}
// if prev button was pressed
elseif (isset($_POST['prev']))
{
echo 'prev';
}
...
?>
I'm trying to retrieve value $GLOBALS['sort'] when < or > button was pressed so I can sort the SQL results by $GLOBALS['sort']. But the variable is undefined when I try to access from outside.