web.php

<?php

$mapReplaces = [
        'ORDER_ID' => 'MARGO'
    ];

    $text
        = '<?xml version="1.0" encoding="utf-8"?> 
<ShipOrderDocument xmlns="http://schemas.quietlogistics.com/V2/ShipmentOrder.xsd" > 
<ClientID>SKITHEEAST</ClientID> 
<BusinessUnit>SKITHEEAST</BusinessUnit> 
<OrderHeader OrderNumber="{ORDER_ID}" OrderType="SO" OrderDate="{ORDER_DATE}" 
ShipDate="{ORDER_DATE}"> 
<Comments>{NOTE}</Comments> 
<ShipMode Carrier="{SHIPPING_TITLE}" ServiceLevel="[Order???]"/>';

    $result = preg_replace_callback('/{(.*?)}/si', function ($match) use ($mapReplaces) {
        if (isset($mapReplaces[$match[1]])) {
            return $mapReplaces[$match[1]];
        }
    }, $text);

    dd($result);