title: Security vulnerabilities with file_get_contents() using variable location
body: <p>Part of my site's application process is that a user must prove ownership of a website. I quickly threw together some code but until now didn't realize that there could be some vulnerabilities with it.</p>
  
  <p>Something like this:</p>
  
  <pre><code>$generatedCode="9s8dfOJDFOIesdsa";
  $url="http://anyDomainGivenByUser.com/verification.txt";
  
  if(file_get_contents($url)==$generatedCode){
  //verification complete!
  }
  </code></pre>
  
  <p>Is there any threat to having a user-provided url for file_get_contents()?</p>
  
  <p>Edit: The code above is just an example. The generatedCode is obviously a bit more elaborate but still just a string.</p>