In my plugin, I'm using UPLOAD_ERR_OK !== $_FILES['form-field-name']['error'] to check for issues with file uploads (from an <input type="file" name="form-field-name" /> in this example).

The UPLOAD_ERR_OK constant in PHP normally is an int, see https://3v4l.org/WjW2j .

However, in WordPress Playground, $_FILES['form-field-name']['error'] returns a string.

Maybe something in