This is a minimal and simple PHP class for collecting prometheus application metrics and storing them in APC used on Proxer.Me

Prometheus.php

<?php
class Prometheus
{
    private static $prefixCounter = 'php_app_counter_';
    private static $prefixGauge = 'php_app_gauge_';
	
	public static function incrementCounter($name)
    {
        self::increment(self::$prefixCounter, $name);
    }

    public static function setGauge($name, $value)
    {
        self::store(self::$prefixGauge, $name, $value);
    }

    private static function increment($prefix, $name)
    {
        if (!function_exists('apcu_fetch')) {
            die("APCu extension is not installed or enabled.");
        }

        $key = $prefix . $name;

        $currentValue = apcu_fetch($key);
        if ($currentValue === false) {
            $currentValue = 0;
        }

        $newValue = $currentValue + 1;
        apcu_store($key, $newValue);
    }

    private static function store($prefix, $name, $value)
    {
        if (!function_exists('apcu_store')) {
            die("APCu extension is not installed or enabled.");
        }

        $key = $prefix . $name;
        apcu_store($key, $value);
    }

    public static function exposeMetrics()
    {
        $counterValues = self::fetchMetrics(self::$prefixCounter, 'counter');
        $gaugeValues = self::fetchMetrics(self::$prefixGauge, 'gauge');

        header('Content-Type: text/plain; version=0.0.4');
        foreach ($counterValues as $counterName => $counterValue) {
            echo "# HELP {$counterName} The total number of hits for {$counterName}\n";
            echo "# TYPE {$counterName} counter\n";
            echo "{$counterName} $counterValue\n";
        }
        foreach ($gaugeValues as $gaugeName => $gaugeValue) {
            echo "# HELP {$gaugeName} The gauge value for {$gaugeName}\n";
            echo "# TYPE {$gaugeName} gauge\n";
            echo "{$gaugeName} $gaugeValue\n";
        }
    }

    private static function fetchMetrics($prefix, $type)
    {
        $metricValues = [];
        foreach (apcu_cache_info()["cache_list"] as $entry) {
            if (strpos($entry['info'], $prefix) === 0) {
                $metricName = substr($entry['info'], strlen($prefix));
                $metricValue = apcu_fetch($entry['info']);
                $metricValues[$metricName] = $metricValue === false ? 0 : $metricValue;
            }
        }

        return $metricValues;
    }

}
?>