How to properly check if PHP is configured correctly to use DOMDocument?

Question

I have a script that uses DOMDocument. In some environments it fails, possibly because a module is not loaded. What I want to do is provide guidance on fixing this issue for users of this script.

Here is a minimal script to reproduce the issue:

<?php
  echo 'start!';
  try {
    $doc = new DOMDocument();
  } catch (Exception $e) {
    echo 'catched';
  }
  echo 'end';
?>

If I open it in a browser (served by my current server, involving Nginx), I only see "start!" (return code 500; if I omit try..catch, the output is the same). So the problem is not only detecting whether the correct module is installed (should I use extension_loaded('dom') to check?), but also that try..catch doesn't seem to work (I don't get caught in the output; in the current case , I'm using PHP 7.4.3).

Do you have any suggestions on how to properly handle this situation?

Answer 1

When a class is not found, an error is raised. This class does not inherit Exception, so your code cannot catch it.

The following code can solve this problem:

try
{
    new DOMDocument();
}
catch(Error $e)
{
    echo 'DOMDocument not available';
}

or:

try
{
    new DOMDocument();
}
catch(Throwable $t)
{
    echo 'DOMDocument not available';
}

Of course, you can directly use extension_loaded('dom') to detect whether the extension is available.

Answer 2

You can use the class_exists() function to test whether a class exists. For example:

if (!class_exists('DOMDocument')){
   echo "Please install DOMDocument";
   exit;
}