If you installed Apache from the repository, it is already safe enough IF you keep it updated.
Most attacks nowadays rely on old vulnerabilities on web applications and brute force attacks via FTP or ssh.
That being said, you can always improve the security of your Apache/PHP with a few best practices, including the PHP security manual and the Apache CIS benchmark:
As the next step, I would recommend monitoring your system. Look at the logs, file changes, etc. The best tools for that are:
You can always monitor your web site remotely, using Network-based Integrity monitoring (that looks for web site changes, whois modifications, blacklisting, etc) using Sucuri (it is a free tool that I developed).