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You can use a logic like this

<?php
        $loggerPath = "../app/logs/sql.log";

        // size greater than 3 MB
        if (filesize($loggerPath) > 3000000) {

            // increment older log files
            for ($i = 10; $i > 0; $i--) {

                if (file_exists('../app/logs/sql.' . $i . '.log')) {
                    rename('../app/logs/sql.' . $i . '.log', '../app/logs/sql.' . ($i + 1) . '.log');
                }
            }

            // delete eleventh file if it exists
            if (file_exists('../app/logs/sql.11.log')) {
                unlink('../app/logs/sql.11.log');
            }

            rename($loggerPath, '../app/logs/sql.1.log');
        }

        $logger = new FileLogger("../app/logs/sql.log", array('mode' => 'a'));

This is only a quick test implementation ;-) It will keep 10 files with max 3 MB size.