How to access a $_SESSION variable inside of modX

<?php echo $_SESSION['product']; ?>

<?php echo $_SESSION['product']; ?>

<?php $_SESSION['product']; ?>

<?php 
return $_SESSION['product'];

$.ajax({
    type: 'POST',
    url: "scripts/processForm1.php",
    cache: false,
    datatype: "html",
    success: function(data){
        …              
    }
});

$.ajax({
    type: 'POST',
    url: "[[snippet-name]]",
    cache: false,
    datatype: "html",
    success: function(data){
        …              
    }
});

<?php
session_start();
require_once 'database_connection.php';
require_once 'clean_up.php';

// Determine if the username is available or not,
// and returns a JSON { "valid": true } or { "valid": false }
$isAvailable = false;

// Get the username from request
$zip = mr_clean($_POST['zip']);
$query = "SELECT * FROM zipcodes WHERE zipcode = ".$zip.";";
$result = mysql_query($query);

if ($result) {
    
    $num_rows = mysql_num_rows($result);
    if($num_rows == 1){
		$location = mysql_fetch_array( $result );
	    $city = $location['city'];
	    $_SESSION['city']= $city;
	    $_SESSION['location']= $location['city'].', '.$location['state'];

	    $isAvailable = true;
    }	
}
echo json_encode(array(
    'valid' => $isAvailable,
	));
?>

<div class="col-md-12">
    <h1>Here are some <?php echo $_SESSION['product']; ?> Insurance Quotes in <?php echo $_SESSION['city']; ?> </h1>
    <h4 id="continue-form">Continue the form below or call (800) 555-5555 to speak to an agent instantly!</h4>
 </div>

session_start();

include $modx->getOption(base_path) . 'scripts/' . $include . '.php';

[[!includeScript? &include=`processForm1`]]

$.ajax({
    type: 'POST',
    url: "[[~99]]",
    cache: false,
    datatype: "html",
    success: function(data){
        …             
    }
});

return $_SESSION[$var];

<div class="col-md-12">
    <h1>Here are some [[!getSessionVar? &var=`product`]] Insurance Quotes in [[!getSessionVar? &var=`city`]] </h1>
    <h4 id="continue-form">Continue the form below or call (800) 555-5555 to speak to an agent instantly!</h4>
 </div>