<?php
$dom = new DOMDocument();
$dom->loadXML("<?xml version='1.0'?><container selected=\"\"/>");
var_dump($dom->documentElement->toggleAttribute('selected'));
echo $dom->saveXML() . PHP_EOL;
var_dump($dom->documentElement->toggleAttribute('selected'));
echo $dom->saveXML();
?>
bool(false)
<?xml version="1.0"?>
<container/>
bool(true)
<?xml version="1.0"?>
<container selected=""/>