rename.php

<?php

$objects = new RecursiveIteratorIterator
(new RecursiveDirectoryIterator(__DIR__),
    RecursiveIteratorIterator::SELF_FIRST);

foreach ($objects as $path => $fileObject) {

    if ( ! is_dir($fileObject)) {
        continue;   
    }

    if ($fileObject->getFileName() == '.' || $fileObject->getFileName() == '..') {
        continue;
    }

    $path = $fileObject->getPathName();

    foreach (scandir($path) as $filename) {
        if ($filename === '.' || $filename === '..') {
            continue
        }
        
        $newname = preg_replace('"\.html.php"', '.phtml', $filename);
        rename($path . '/' . $filename, $path . '/' . $newname);
    }
}