HDWebmobile Formula Pricing

Description

HDWebmobile Formula Pricing lets customers enter measurements or quantities on the product page, and prices the product live from a formula you write using those field names (e.g. 10 + width * height * 0.05 for a custom-cut material priced by area). This is a common real need for stores selling custom-sized fabric, framed prints, engraved items, or anything priced by dimension or quantity, not just a flat number.

Security by design

The simplest way to implement formula-based pricing — passing the merchant’s formula text straight into PHP’s eval() at calculation time — is also the most dangerous: it turns any formula input into a remote code execution path. This plugin closes that entire vulnerability class by construction:

• The formula is tokenized character-by-character into a strict set of allowed pieces: numbers, your own configured field names, and the four arithmetic operators plus parentheses. Anything else in the formula text — any other letter sequence, any function-call-looking pattern, any special character — causes the formula to be rejected outright when you save it.
• The token stream is evaluated by a small hand-written arithmetic parser. There is no eval(), no create_function(), no dynamic function dispatch of any kind anywhere in this plugin — the only operations that can ever run are addition, subtraction, multiplication, and division of plain numbers.
• The customer-facing live price preview also never exposes your formula to the browser: it’s computed by an AJAX call back to your own server, through the exact same safe evaluator used at checkout, so the formula text itself is never sent to a visitor’s browser.
• The real price used at checkout is always recomputed server-side from the product’s own stored fields and formula — never trusted from what the browser submitted.

Key Features

• Add up to 6 numeric input fields per product (label, variable name, min/max bounds, default value)
• Write a formula referencing those fields by name — validated for safety the moment you save it
• Live price preview on the product page as the customer types, computed server-side
• Optional minimum/maximum price clamps, so no combination of inputs can produce an unreasonable price
• Every field’s min/max bounds are enforced again server-side at add-to-cart time, not just in the browser
• Field values show on the cart, checkout, and order screens exactly as entered

Limitations (please read before installing)

• Simple products only in this version — no variable-product support
• Formulas support only +, -, *, /, and parentheses (no exponents, square roots, or conditional logic) — deliberately, since a richer grammar is a larger safety surface to get right
• No formula “test” button in the admin beyond the save-time validation — if a formula is rejected, the error message names exactly what’s wrong (an unknown variable, a bad character, division by zero with the values you tried)
• Requires a Regular Price to be set on the product (even a nominal placeholder) — this is standard WooCommerce behavior for any product with no price at all, not something this plugin can work around

Enjoying this plugin?

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How to Use

1. Add your input fields

On a simple product’s “Formula Pricing” tab, click “+ Add Field” for each number the customer should enter (e.g. “Width in cm” with variable name width). Set a minimum, an optional maximum, and a default value for each.

2. Write the formula

In the Formula box, write an expression using your field names exactly as shown, e.g. 10 + width * height * 0.05. Save the product — if the formula uses anything other than your field names, numbers, and + - * / ( ), you’ll see exactly what’s wrong and the formula won’t be saved until it’s fixed.

3. What the customer sees

The product page shows your input fields above the Add to Cart button, with a live estimated price that updates as they type. The real price is confirmed the moment the product is actually added to the cart.

4. Keeping prices sane

Set an optional minimum and/or maximum price on the same tab — no matter what a customer enters (within their allowed field ranges), the final price is clamped to stay within those bounds.