PHP Script to pull PDFs from an API

General information for the website: Script to pull PDFs from FreeAgent
Kind of development: Customization of existing website
Description of requirements/features: FreeAgent is an online accounting system.

Within FreeAgent you can raise invoices.

Each month, I need to print all Invoices raised within that month.

There is a PHP script available on GitHub that makes use of the FreeAgent API to easily show all invoices within a given time period and provide direct download links to each invoice.

I need this extending to give me more options.

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Phase 1

Extend script to offer two choices.
1> List individual invoices (this is what it does now)
2> Bulk print invoices

If the second option is selected, each invoice within the given period will be combined into single, multi-page PDF document.

E.g. 1> List individual invoices (the script does this already)
Invoice A -> Download
Invoice B -> Download
Invoice C -> Download
Invoice D -> Download
Invoice E -> Download

E.g. 2> Bulk print invoices
Download all invoices for this period (single PDF containing Invoice A, Invoice B, Invoice C, Invoice D and Invoice E and separate pages)

Phase 2

Add checkbox options to allow me to select "all invoices", "open or overdue invoices", "draft invoices"
These are options from within the FreeAgent API. I.e. when creating an invoice it will be categorised into one of these labels. So when querying the API you will need to pass one of these options to return just the invoices requested.

E.g. checking "draft invoices" will only return draft invoices created within the period specified

Phase 3

FreeAgent are updating their API at the end of the month. Any changes made to this existing PHP script should take this into account

Useful links
PHP script on Github -> https://github.com/enru/fa-invoices
FreeAgent API -> https://dev.freeagent.com/
Change to FreeAgent API -> http://engineering.freeagent.com/2015/10/09/weak-dh/
Extra services needed: Customization, 3rd party integration
Extra notes: