example.php

<?php 

// Usage example

$testsRunner = new TestsRunner();
$testsRunner->setJsonFileLocation('path/to/tests.json');
$values = $testsRunner->getValues(37, 20, 'us');

tests.json

[
	{
		"name": "Example Test",
		"conditions": {
			"product_id": "0",
			"template_id": "0",
			"country": "us",
			"query": {
				"template": "main",
				"id": "77"
			}
		},
		"plans": [
			{
				"ratio": 25,
				"values": {
					"installer": "setup.exe",
					"language": "en"
				}
			},
			{
				"ratio": 40,
				"values": {
					"installer": "setup2.exe",
					"size": "large",
					"language": "ru"
				}
			},
			{
				"ratio": 35,
				"values": {
					"installer": "setup3.exe",
					"size": "small",
					"language": "de"
				}
			}
		]
	},
	{
		"name": "Another Test",
		"conditions": {
			"product_id": "37",
			"template_id": "21",
			"country": "ru",
			"query": null
		},
		"plans": [
			{
				"ratio": 23,
				"values": {
					"param1": "1"
				}
			},
			{
				"ratio": 75,
				"values": {
					"param1": "2"
				}
			},
			{
				"ratio": 2,
				"values": {
					"param1": "3"
				}
			}
		]
	}
]

TestsRunner.php

<?php


class TestsRunner
{

    private $jsonFilePath;


    public function setJsonFileLocation($path)
    {
        if (!file_exists($path)) {
            throw new Exception(sprintf('JSON file "%s" was not found', $path));
        }

        $this->jsonFilePath = $path;
    }


    public function getValues($productId, $templateId, $countryCode)
    {

        if (!$productId) {
            throw new Exception('productId parameter is required');
        }

        if (!$templateId) {
            throw new Exception('templateId parameter is required');
        }

        if (!$countryCode) {
            throw new Exception('countryCode parameter is required');
        }

        try {

            $test = $this->findFirstMatchedTest($productId, $templateId, $countryCode);
            $winnerPlan = $this->runTest($test);

            return $winnerPlan['values'];
        }

        catch (Exception $e) {
            return array();
        }
    }


    private function findFirstMatchedTest($productId, $templateId, $countryCode)
    {
        $tests = $this->getTestsFromFile();

        foreach ($tests as $test) {

            if ($this->matches($test, $productId, $templateId, $countryCode)) {
                return $test;
            }

        }

        throw new Exception('No tests matched');
    }


    private function getTestsFromFile()
    {
        return json_decode(file_get_contents($this->jsonFilePath), true);
    }


    private function runTest($test)
    {
        $value = mt_rand(0, 100);
        $winnerIndex = 0;
        $min = 0;

        foreach ($test['plans'] as $index => $plan) {

            $planRatio = (int)$plan['ratio'];

            $max = $min + $planRatio;

            if ($value >= $min && $value <= $max) {
                $winnerIndex = $index;
                break;
            }

            $min += $planRatio;
        }

        return $test['plans'][$winnerIndex];
    }


    private function matches($test, $productId, $templateId, $countryCode)
    {
        $conditions = $test['conditions'];

        // if there is product constrain in test
        if ($conditions['product_id']) {
            // and product not matches
            if ($productId != $conditions['product_id']) {
                return false;
            }
        }

        // if there is template constrain in test
        if ($conditions['template_id']) {
            // and template not matches
            if ($templateId != $conditions['template_id']) {
                return false;
            }
        }

        // if there is query constrain in the test
        if ($conditions['query']) {

            foreach ($conditions['query'] as $paramName => $paramValue) {

                if (!isset($_GET[$paramName])) {
                    return false;
                }

                if ($_GET[$paramName] != $paramValue) {
                    return false;
                }
            }
        }

        // if there is country constrain in the test
        if ($conditions['country']) {

            if (strtolower($countryCode) !== strtolower($conditions['country'])) {
                return false;
            }
        }

        return true;
    }


}