How to convert base64 encoded attachment to file (PDF, msg, eml) - php

Question

We have an XML file exported from ServiceNow which we are trying to import into our custom PHP app.
Each attachment <sys_attachment> are split into chunks <sys_attachment_doc> which is ordered by the <position> element.
<sys_attachment>
<chunk_size_bytes>734003</chunk_size_bytes>
<compressed>true</compressed>
<content_type>application/pdf</content_type>
<encryption_context display_value="" />
<file_name>Filename.pdf</file_name>
</sys_attachment>
<sys_attachment_doc>
<data>[BASE64 ENCODED STRING HERE]</data>
<length>[STRING LENGTH]</length>
<position>0</position>
</sys_attachment_doc>
<sys_attachment_doc>
<data>[BASE64 ENCODED STRING HERE]</data>
<length>[STRING LENGTH]</length>
<position>1</position>
</sys_attachment_doc>
We have tried combining the string and base64_decoding it but to no avail.
<?php
header('Content-type: application/pdf');
header('Content-Disposition: attachment; filename="servicenow.pdf"');
//echo base64_decode($chunk0.$chunk1);
echo base64_decode($chunk0).base64_decode($chunk1);
?>
We are unable to find any documentation on how to convert these attachments to files outside of ServiceNow (PHP). Is there an extra step that needs to be done before decoding the string and converting to file (PDF)
Edit: I manage to solve it using #Joey answer. I base64_decode the chunks then afterwards combine it. The combined string is actually gzip compressed. We used gzdecode() to generate the PDF.
$attachment = base64_decode($chunk0).base64_decode($chunk1);
echo gzdecode($attachment);

Answer 1

One thing that may be tripping you up is that <compressed> flag. Since that's reading as true, the data is also gzipped, so attachments start from byte[], which then get gzipped, broken into chunks, and base64 encoded (per chunk!).
I don't know how to do this in php specifically, but this strategy should work:
Base64 decode of each chunk will give you a byte[] per chunk.
Combine those chunks in order of position to give you one big byte stream
gunzip that stream into another big byte stream which should be your file.