dbchecker.php

<?php

$mysql_checker = function(){
    $cfg = explode("\t", DB_CFG);
    error_reporting(E_ERROR);
    $try_num = 20;
    $try_interval = 500000; // 0.5 sec

    foreach(range(0, $try_num) as $times)
    {
        $mysqli = new mysqli($cfg[2], $cfg[0], $cfg[1], $cfg[3]);

        // connect fail
        if($mysqli->connect_error) {
            if($times === ($try_num - 1))
            {
                header($_SERVER['SERVER_PROTOCOL'] . ' 500 Internal Server Error', true, 500);
                echo file_get_contents(ROOT_PATH . '/static/500.html');
                exit;
            }
            usleep($try_interval);
            continue;
        }

        // connected
        $mysqli->close();
    }
};
$mysql_checker();
unset($mysql_checker);