One of the easiest ways to improve an auction system is by preventing bidders from increasing the current price by tiny amounts. On professional auction platforms like eBay, each new bid must exceed the current bid by a minimum increment.

In this lesson, we’ll implement the same concept in our Flipnzee Auctions plugin by introducing a Minimum Bid Increment. This ensures every new bid is meaningful and keeps the auction moving smoothly.

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Why Is a Minimum Bid Increment Important?

Imagine an auction with a current bid of $1,000.

Without validation, users could place bids like:

• $1,000.01
• $1,000.02
• $1,000.03

Technically, each bid is higher, but they barely advance the auction. This can frustrate bidders and unnecessarily prolong the bidding process.

By requiring a minimum increment—for example, $10—the next valid bid would have to be at least $1,010.

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What We’ll Build

At the end of this lesson, bidders will only be allowed to submit bids that satisfy:

New Bid >= Current Bid + Minimum Increment

For example:

Current Bid	Minimum Increment	Minimum Allowed Bid
$100	$5	$105
$250	$25	$275
$1,000	$50	$1,050

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Step 1: Define a Minimum Increment

We’ll begin with a fixed increment.

$minimum_increment = 10;

Later, we’ll make this configurable from the WordPress admin.

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Step 2: Calculate the Minimum Allowed Bid

Instead of checking only whether the bid is greater than the current bid, we’ll calculate the next valid amount.

Example:

$minimum_allowed_bid =
    $current_bid +
    $minimum_increment;

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Step 3: Validate the Bid

The bidding logic will reject bids below the minimum requirement.

Conceptually:

if (
    $bid_amount <
    $minimum_allowed_bid
) {

    return false;

}

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Step 4: Show a Helpful Error Message

Instead of silently rejecting the bid, inform the user why it failed.

Example:

Your bid must be at least $1,010.

Helpful validation messages create a much better user experience.

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Step 5: Display the Minimum Bid

Near the bid input box, we’ll display information such as:

Minimum Bid:
$1,010

This tells visitors exactly what they need to bid.

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Step 6: Prevent Invalid Form Submission

We’ll add HTML attributes so users cannot enter values below the minimum amount.

For example:

<input
type="number"
min="1010"
step="10">

This provides immediate feedback before the form is even submitted.

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Expected Result

Instead of allowing any value above the current bid:

Current Bid:
$1,000

Your Bid:
1001 ❌

Place Bid

Visitors will see:

Current Bid:
$1,000

Minimum Bid:
$1,010

Your Bid:
1010 ✔

Place Bid

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What We’ll Learn

In this lesson, you’ll learn how to:

• Calculate dynamic minimum bid values
• Validate bids on the server
• Improve user experience with meaningful validation messages
• Add client-side validation using HTML attributes
• Prepare the plugin for configurable bidding rules

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Why This Matters

Professional auction platforms don’t simply accept any higher bid—they enforce bidding rules that keep auctions fair, competitive, and easy to follow.

By adding a minimum bid increment, Flipnzee Auctions becomes significantly more professional and prevents users from placing trivial bid increases.

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Assignment

Before moving to Lesson 47, try the following:

• Set the minimum increment to $10.
• Verify that bids below the required amount are rejected.
• Confirm that valid bids are still accepted and saved.
• Display the minimum required bid below the current bid so visitors always know the next valid amount.

In the next lesson, we’ll take this a step further by making the minimum bid increment configurable from the WordPress admin, allowing different auction strategies without changing any code.

• Category: Building Flipnzee Auction