Barebones PHP error logging class

Log.php

<?php
// src/Log.php
namespace App;

/**
 * Barebones error logging class
 * 
 * Location: <project root>/src/Log.php
 * 
 * Usage: 
 * App\Log::error('Your error message');
 * 
 * Output (<project root>/storage/logs/error.log):
 * [2022-04-02 16:20:46] Your error message
 */
class Log
{
    private const PATH = __DIR__ . '/../storage/logs';
    private const FILENAME = 'error.log';

    private static self|null $singleton = null;

    protected function __construct()
    {
        self::createStorageFolderIfNotExists();
    }

    public static function singleton(): ?Log
    {
        if (self::$singleton === null) {
            self::$singleton = new self;
        }

        return self::$singleton;
    }

    public static function error(string $message): void
    {
        (new self)->writeError($message);
    }

    private static function createStorageFolderIfNotExists(): void
    {
        if (!file_exists(self::PATH)) {
            mkdir(self::PATH, 0755, true);
        }
    }

    private function writeError(string $message): void
    {
        $line = '['. self::timestamp() . '] ' . $message;
        $filename = self::PATH . DIRECTORY_SEPARATOR . self::FILENAME;
        file_put_contents($filename, $line, FILE_APPEND);
    }

    private static function timestamp(): string
    {
        return date('Y-m-d H:i:s');
    }
}