To block the Bytespider bot on your website using PHP, you can add the following code to the beginning of the .htaccess file on your server:

```
RewriteEngine On
RewriteCond %{HTTP_USER_AGENT} Bytespider [NC]
RewriteRule .* - [F,L]
```

This code checks the HTTP_USER_AGENT header for the presence of the string "Bytespider" and, if found, returns a 403 error (Forbidden), effectively blocking the Bytespider bot's access to your site.

Please note that you may need access to the .htaccess file on your server to use this method. Make sure you understand how this code works before making any changes to your server.