title: Disallow variable overwrite in PHP
body: <p>Is it possible to prevent variables from being overwritten in PHP?  I am making a system that has some reserved variables and I don't want them to be replaced with something else after a certain point.  It is possible? If not, what can I do to approach something close to this?</p>
  
  <p>Some of these vars are instantiated classes so I can't define them as constants.</p>