PHP developer to make some changes in the existing php based application

I need my current PHP-based web application (running locally on XAMPP with a MySQL backend) customization. customization are as following:
- sales and purchase document fully meets ZATCA Phase 2 e-invoicing rules in Saudi Arabia. Integration tool and codes are available to handle both B2B and B2C flows end-to-end, including credit and debit notes, without breaking the existing UI or database structure.

What has to change using the existing both the code:
• Forms: add all mandatory ZATCA data elements, automate the VAT and total calculations, and store the hash, digital signature, QR code string and any other Phase 2-specific fields that ZATCA requires.
- in the configuration page, add a tab named "EGS-ZATCA" and set permissions in the db same as the existing other functions (not complicated, its very easy- you can understand after seeing the existing app)
• Invoice logic: generate compliant XML payloads along with the on-screen / PDF format, and keep the workflow identical for B2B and B2C users using the available tools.
• Notes: enable automated creation of credit and debit notes for returns or corrections, again for both customer types, linked to the original invoice.
• Reporting: build interactive reports for invoices and notes with on-screen filters plus one-click export to Excel and PDF. They must show status (draft, cleared, cancelled), totals, VAT amounts and reference numbers exactly as ZATCA expects.
• Data integrity: keep everything stored in MySQL and ensure it works on the same local XAMPP stack.

Deliverables (ultraviewer remote only due to privacy reasons)
1. Updated PHP source files and SQL scripts.
2. A brief setup guide so I can redeploy on my local PC.
3. Test data demonstrating a successful B2B invoice, B2C invoice, credit note and debit note, each validated against ZATCA Phase 2 rules.

I’ll provide ultraviewer access to the system where the codes are installed for the customization, codebase and DB schema once we agree on the approach; please note that the timeframe for these changes is only one day from confirmation since 90% of codes and inegration codes/tools are already available.