- array_map: documentation ( source)
- str_replace: documentation ( source)
- compact: documentation ( source)
<?php
$str = "We have received ##AMOUNT## ##CURRENCY## for your OrderID n. ##ORDER_ID##";
$AMOUNT = 123;
$CURRENCY = 'GBP';
$ORDER_ID = 20123;
$find = ['AMOUNT', 'CURRENCY', 'ORDER_ID'];
echo str_replace(
array_map(function($v){ return '##'.$v.'##'; }, $find),
compact(...$find),
$str
);