Description:
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Sometimes the DOMXpath->evaluate() function returns unreliable results.  This seems to happen when the document has at least a depth of 3 and selectors are used at multiple levels.  There may be other conditions that trigger it; I'm not sure.  The test script below is consistently showing errors for me.

Test script:
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<?php
$rawxml = '<?xml version="1.0"?><root><field attr1="val1"><field attr1="val1"/></field></root>';
$rawxpath = "/root[1]/field[@attr1='val1']";

$results = array();
for ($i = 0; $i < 10000; $i++) {
        $xmlDocument = new DOMDocument();
        $xmlDocument->loadXML($rawxml);
        $xp = new DOMXPath($xmlDocument);
        $xpr = $xp->evaluate($rawxpath, $xmlDocument->documentElement);

        if (isset($results[(string) $xpr->length]))
                $results[(string) $xpr->length]++;
        else
                $results[(string) $xpr->length] = 1;
}

foreach ($results as $matches => $count)
        printf("In %5d instances, found $matches match(es) for $rawxpath\n", $count);

Expected result:
----------------
In 10000 instances, found 1 match(es) for /root[1]/field[@attr1='val1']

Actual result:
--------------
The exact numbers seem to vary, but I get roughly this response:


In  9924 instances, found 1 match(es) for /root[1]/field[@attr1='val1']
In    76 instances, found 2 match(es) for /root[1]/field[@attr1='val1']