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You cannot just download a list of all domains and email addresses. All a DNS cache does is save DNS lookup results to avoid hitting the upstream servers for common lookups. Windows has one built-in already (see: ipconfig /displaydns).

You're generally not going to find such specific examples in a manual. You need to use the documentation and examples given to learn the components so you can then put them together as needed to accomplish your task. Perhaps you should lookup what getAttribute does. You might also need to read up on how the DOM (the link talks JavaScript, but the interfaces work the same in any language) works in general if you want to continue using DOMDocument. Spend some time reading the documentation for DOMDocument and some about the DOM and you should be able to figure out where you went wrong in your above attempts.