title: file_exists() works on string but not string variable
body: <p>I have problem with function file_exists() in PHP. Result of code below is always "Exists on string", but in my opinion it should print two messages. </p>
  
  <pre><code>$file = 'test_file.txt';
    if (file_exists($file)){
         echo 'Exists on variable';
    }
    if (file_exists('test_file.txt'){
         echo 'Exists on string';
    }
  </code></pre>