composer.json

{
    "require": {
        "hab/picarecord": "^1.1",
        "hab/picareader": "^1.2"
    }
}

fetch-pica.php

<?php

use HAB\Pica\Record\Record;
use HAB\Pica\Reader\PicaNormReader;

require_once __DIR__ . '/vendor/autoload.php';

$ppn = '1018291776';

$psiTemplate = 'https://kataloge.uni-hamburg.de/DB=1/PLAIN=Y/CHARSET=UTF-8/PLAINTTLCHARSET=UTF-8/PPN?PPN=%s';
$psiUrl = sprintf($psiTemplate, $ppn);
$content = file_get_contents($psiUrl);
        
$reader = new PicaNormReader();
$reader->open($content);

$record = $reader->read();

$fields = $record->getFields('201@');
print_r($fields);