SPL Autoload

<?php
function autoload($className) {
    $className = ltrim($className, '\\');
    $fileName = '';
    $namespace = '';
    if ($lastNsPos = strrpos($className, '\\')) {
        $namespace = substr($className, 0, $lastNsPos);
        $className = substr($className, $lastNsPos + 1);
        $fileName = str_replace('\\', DIRECTORY_SEPARATOR, $namespace) . DIRECTORY_SEPARATOR;
    }
    $fileName .= str_replace('_', DIRECTORY_SEPARATOR, $className) . '.php';

    $ret = strtolower($fileName);
    if (file_exists($ret)) {
        require $fileName;
    }
    else {
        $db = debug_backtrace()[1];
        $fn = basename($ret);
        $dn = __DIR__ . DIRECTORY_SEPARATOR . dirname($ret);
        $file = $db["file"];
        $line = $db["line"];
        ob_start();
        ?>
        <div>
            Couln't load class <b><?php echo "$namespace\\$className" ?></b> |
            "<b><?php echo $fn ?></b>" not found in
        </div>
        <div><b><?php echo $dn ?></b></div>
        <div><b><?php echo $file ?></b> on line <b><?php echo $line ?></b></div>
        <?php
        die(preg_replace("/\r|\n|\\s\\s+/", "", ob_get_clean()));
    }
}

spl_autoload_register('autoload');