Implementing WeChat Red Packet Random Distribution in PHP

Inspired by a spontaneous red‑packet giveaway in a WeChat alumni group, the author investigates how WeChat distributes a total of 10 CNY among 28 random recipients.

The discussion references a popular Zhihu post about the algorithm but notes the lack of official details.

Key Requirements for the Algorithm

Every participant must receive some amount.

The sum of all received amounts must equal the total amount.

Amounts should vary but not be excessively disparate.

The algorithm must remain simple.

Before coding, a progressive model is built to understand the pattern.

<code>Set total = 10 CNY, N participants receive randomly: N=1: amount = X N=2: first amount = random(0.01, 9.99), second = total - first N=3: first = random(0.01, 0.98) second = random(0.01, total - first - 0.01) third = total - first - second ... (continue similarly for larger N)</code>

Initial implementation uses a simple random bound:

<code>header("Content-Type: text/html;charset=utf-8"); $total = 10; // total amount $num = 8; // number of red packets $min = 0.01; // minimum per person for ($i = 1; $i < $num; $i++) { $safe_total = $total - ($num - $i) * $min; // safe upper bound $money = mt_rand($min * 100, $safe_total * 100) / 100; $total -= $money; echo "Packet $i: $money 元, remaining: $total 元<br/>"; } echo "Packet $num: $total 元, remaining: 0 元"; </code>

This version produces large fluctuations, making the distribution uninteresting.

To reduce variance, the safe upper bound is adjusted to the average of the remaining amount:

<code>header("Content-Type: text/html;charset=utf-8"); $total = 10; $num = 8; $min = 0.01; for ($i = 1; $i < $num; $i++) { $safe_total = ($total - ($num - $i) * $min) / ($num - $i); $money = mt_rand($min * 100, $safe_total * 100) / 100; $total -= $money; echo "Packet $i: $money 元, remaining: $total 元<br/>"; } echo "Packet $num: $total 元, remaining: 0 元"; </code>

The improved algorithm yields more balanced results, as shown in the output screenshot.