$xc->findnodes('//*/info/fooTransaction/transactionDetail/histFile:transactionSummary/*')

##</code><code>##

# looking by local-name
$xc->findnodes('//*/info/fooTransaction/transactionDetail/*[local-name() = "transactionSummary"]/*');

# looking by qualified name
$xc->findnodes('//*/info/fooTransaction/transactionDetail/*[name() = "histFile:transactionSummary"]/*');

##</code><code>##

$doc->findnodes('//*/info/fooTransaction/transactionDetail/*[name() = "histFile:transactionSummary"]/*');