// Created on savesnippets.com · https://savesnippets.com/R0PrO952xhOhd5
<?php
function rotatingLog(string $path, string $line, int $maxBytes = 5_000_000, int $keep = 3): void {
    if (is_file($path) && filesize($path) >= $maxBytes) {
        for ($i = $keep; $i > 1; $i--) {
            $older = "$path." . ($i - 1);
            if (is_file($older)) rename($older, "$path.$i");
        }
        rename($path, "$path.1");
    }
    file_put_contents($path, $line . PHP_EOL, FILE_APPEND | LOCK_EX);
}

rotatingLog('/var/log/myapp.log', '[' . date('c') . '] something happened');