php 58 lines · 8 steps

How a feature flag evaluator decides

A flag evaluator resolves an on/off decision from static config, targeting rules, and a stable percentage rollout.

Explained by highlit

Intermediate feature-flags rollout hashing pattern-matching configuration

1<?php
2 
3namespace App\FeatureFlags;
4 
5class FlagEvaluator
6{
7    public function __construct(private array $flags)
8    {
9    }
10 
11    public function isEnabled(string $flag, array $context = []): bool
12    {
13        $config = $this->flags[$flag] ?? null;
14 
15        if ($config === null || ($config['enabled'] ?? false) === false) {
16            return false;
17        }
18 
19        foreach ($config['rules'] ?? [] as $rule) {
20            if ($this->matchesRule($rule, $context)) {
21                return $rule['serve'] ?? true;
22            }
23        }
24 
25        $rollout = $config['rollout'] ?? 100;
26 
27        return $this->withinRollout($flag, $context['user_id'] ?? '', $rollout);
28    }
29 
30    private function matchesRule(array $rule, array $context): bool
31    {
32        $value = $context[$rule['attribute']] ?? null;
33 
34        return match ($rule['operator']) {
35            'in'          => in_array($value, $rule['values'], true),
36            'not_in'      => !in_array($value, $rule['values'], true),
37            'equals'      => $value === $rule['value'],
38            'greater_than' => is_numeric($value) && $value > $rule['value'],
39            'ends_with'   => is_string($value) && str_ends_with($value, $rule['value']),
40            default       => false,
41        };
42    }
43 
44    private function withinRollout(string $flag, string $userId, int $percentage): bool
45    {
46        if ($percentage >= 100) {
47            return true;
48        }
49 
50        if ($percentage <= 0 || $userId === '') {
51            return false;
52        }
53 
54        $bucket = crc32($flag . ':' . $userId) % 100;
55 
56        return $bucket < $percentage;
57    }
58}

01 / 01

STEP 01

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Three takeaways

1Layering a global switch, targeting rules, and a percentage rollout lets one method serve nuanced flag decisions.
2Hashing a stable key like flag plus user id gives consistent bucketing so a user always lands the same way.
3The null-coalescing operator and match expression keep config-driven logic terse and safe against missing keys.

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