Reddit.com: What non-PHP stuff should a PHP developer know?

On Reddit.com there's a good discussion going on to answer the question "What non-PHP stuff should a PHP developer know?"

I was looking at job description for a web developer, and one of the big responsibilities was database maintenance. [...] And along those lines, what other skills would be useful for a PHP developer to have that aren't directly PHP-based?

Suggestions so far include things like:

• Linux command-line skills
  
• some Python
  
• Virtualization (making and administering VMs)
  
• Version control systems (svn or git)
  
• Database architecture
  
• HTML/CSS/Javascript