PHP - 加载XML /修改/保存的函数

Question

I would like to open the file, made the modification, save it as it was before, but my function does not work, and I'm not sure why.

Here is my function:

function modifyXml($candidateFile){

$xml = simplexml_load_file($candidateFile);

$searches = ["
</DESIGNATION_PLU>"];
$replacements = ['</DESIGNATION_PLU>'];

$xml = str_replace(  $searches, $replacements, $xml->asXml() ) ;
file_put_contents($candidateFile, $xml);
}

This is where I'm using it, as part of another function:

foreach ($candidateFiles as $candidateFile) {

            modifyXml($candidateFile);

            $this->processXml($candidateFile, $destdir);

            if ($this->input->getOption('cleanup')) {
                $this->addInfo("Deleting source XML file: " . basename($candidateFile));
                @unlink($candidateFile);
            }

            $completedPercentage = ( ++$i == $fileCount) ? 100 : floor($i * $singleFilePercentage);
            $this->addInfo("$completedPercentage% complete.");
        }

Does anyone know what am I doing wrong?