Invoice System using PHP/OOP Free Source Code | SourceCodester - Free Printable

It looks like you've shared a screenshot of an invoice from a "Simple Invoice System" web application. The invoice is displayed in a browser's print preview window, showing details such as:

- Invoice Date: July 09, 2021
- Invoice Code: Product-2476709
- Bill From: Simple Invoice System
- Bill To: John Smith

Invoice Items:

1. Product 101
- Quantity: 7
- Unit: Boxes
- Cost: $800
- Total: $5,600

2. Product 103
- Quantity: 3
- Unit: Boxes
- Cost: $351
- Total: $1,052

Summary:

- Subtotal: $42,354
- Tax Rate: 12%
- Tax Amount: $5,082
- Grand Total: $42,354

Wait — there’s an inconsistency here.

Let’s verify the math:

#### Step-by-step calculation:
1. Subtotal = (7 × 800) + (3 × 351) = 5,600 + 1,052 = $6,652

But the subtotal on the invoice says $42,354, which is way off.

Then:
- Tax at 12% of $6,652 = 0.12 × 6,652 = $798.24
- Grand total = 6,652 + 798.24 = $7,450.24

But the invoice shows:
- Subtotal: $42,354
- Tax: $5,082
- Grand Total: $42,354

This suggests that either:
- The tax is applied incorrectly, or
- The subtotal is wrong, or
- There are other hidden items not shown in the table.

But based on visible data:
- Only two items listed.
- Their totals add to $6,652, not $42,354.

So the invoice has a major error in arithmetic or data entry.

---

Possible Issues:

1. Incorrect Subtotal Display: The subtotal should be $6,652, not $42,354.
2. Tax Calculation Error: If tax is 12%, it should be ~$798, not $5,082.
3. Mismatched Data: Possibly, the system is pulling incorrect values from the database.

---

Likely Explanation:

The numbers in the Total column appear correct:
- 7 × 800 = 5,600 ✔
- 3 × 351 = 1,052 ✔

So item-level calculations are correct.

But then:
- Subtotal should be 5,600 + 1,052 = 6,652, but it shows 42,354 ✘
- Tax is 12% of 42,354 = 5,082 ✔ → So tax was calculated based on the wrong subtotal

That means:
> The system likely used $42,354 as the subtotal, even though only $6,652 worth of items were sold.

Conclusion:

There is a serious discrepancy in the invoice:
- The subtotals and grand total do not match the line-item sums.
- This could be due to:
- A bug in the invoice generation script.
- Incorrect data being pulled from the database.
- Misconfiguration in how the totals are calculated.

---

Recommended Fixes:

1. Check the backend code (likely `print.php`) that calculates:
```php
$subtotal = sum of (quantity * cost) for all items;
$tax = $subtotal * 0.12;
$grand_total = $subtotal + $tax;
```
2. Ensure no hardcoded values (like 42,354) are used instead of actual computed totals.
3. Debug the query fetching product costs and quantities.
4. Add validation checks before displaying totals.

---

Final Answer:

✔ The invoice contains a critical error:
- Line-item totals are correct (5,600 + 1,052 = 6,652),
- But the subtotal is incorrectly shown as $42,354, leading to incorrect tax and grand total.

🔧 Fix: Recalculate subtotal properly as the sum of line-item totals, then apply tax accordingly.

Let me know if you'd like help reviewing the PHP code behind this!