Prova detta:
<?php
// fetch the very last entry
$Barcode = 'PU000001234567';
preg_match("/(\D+)(\d+)/", $Barcode, $Matches); // Matches the PU and number
$ProductCode = $Matches[1];
$NewID = intval($Matches[2]);
$NewID++;
$BarcodeLength = 12;
$CurrentLength = strlen($NewID);
$MissingZeros = $BarcodeLength - $CurrentLength;
for ($i=0; $i<$MissingZeros; $i++) $NewID = "0" . $NewID;
$Result = $ProductCode . $NewID;
echo $Result;
// insert into database with $Result