I found the answer above to be helpful, but it didn’t quite work for me.
This ended up working better:
// Gets rid of all namespace definitions
$xml_string = preg_replace('/xmlns[^=]*="[^"]*"/i', '', $xml_string);
// Gets rid of all namespace references
$xml_string = preg_replace('/[a-zA-Z]+:([a-zA-Z]+[=>])/', '$1', $xml_string);