<p>I would like to use my laptop as a web development (PHP, Python, etc.) machine, but I'm hesitant to do this unless I can ensure that it can not be accessed by the outside world.</p>

<p>I'm guessing that something more than a firewall is necessary, such as configurations to the Apache configuration files, but I'm not sure what else I would need to be 100% sure it's locked down tightly.</p>

You need to configure the server daemon to only bind to localhost using the Listen directive like this:
```
```
Listen 127.0.0.1
```
```
An alternative is to configure access control for the main server like this
```
```
<Directory "/var/www/localhost/htdocs">
AllowOverride None
Deny from all
Allow from 127.0.0.1/255.0.0.0
</Directory>
```
```
Remember to put the root directory of your server in the Directory Directive.