title: Using DIRECTORY_SEPARATOR in file_exists() on Windows
body: <pre><code>var_dump(DIRECTORY_SEPARATOR) // string '\' (length=1)
  var_dump(file_exists("C:/1212.txt")); // true
  var_dump(file_exists("C:\1212.txt")); // false
  var_dump(file_exists("C:".DIRECTORY_SEPARATOR."1212.txt")); // false
  </code></pre>
  
  <p>What does <code>DIRECTORY_SEPARATOR</code> do? Why is the above case false when using <code>DIRECTORY_SEPARATOR</code>?</p>