Load external XML and save it using PHP

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I want to load and save an external \[code\]XML\[/code\] file that changes continuouslye.g. "http://something.com/file.xml"Sometimes the file is not available because server is overloaded or down and I want to use this file instead and re-save it if the next load is available also let people know the file that they are reading is a backup file because the original is not available.I read about simplexml_load_file and DOMDocument and I don't know which one is the better solution.