how to correctly append xml child elements in php

Solution:

Thenew SimpleXMLElement thenasXML() is the issue.

addChild is enough to make the element then its return value is the node, then subsequent addChild's on the node make the children.

<?php
$root = new SimpleXMLElement('<movies/>');
$movies = ['foo', 'bar'];
foreach($movies as $movie_name) {
    $movie = $root->addChild('movie');
    buildMovieTag($movie, $movie_name);
}

function buildMovieTag($node, $name) {
    $node->addChild('name', $name);
}

echo $root->saveXML();
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<?xml version="1.0"?>
<movies><movie><name>foo</name></movie><movie><name>bar</name></movie></movies>
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Edit: if your data is more extensive, structure it so its standard key/value array then loop over the items. The actual issue to the original question is noted on the first line of the answer.

<?php
$root = new SimpleXMLElement('<movies/>');
$movies = [
    ['name'=>'foo', 'released' => '2020-12-01 00:00:00', 'plot' => 'foo is a foofoo'],
    ['name'=>'bar', 'released' => '2020-12-02 00:00:00', 'plot' => 'bar is a barbar'],
];
foreach ($movies as $movie) {
    buildMovieTag($root->addChild('movie'), $movie);
}

function buildMovieTag($node, $data) {
    foreach ($data as $key => $item) {
        // just year from datetime string
        if ($key === 'released') $item = date_create($item)->format('Y');
    
        $node->addChild($key, $item);
    }
}

echo $root->saveXML();
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Result (manual formatted)

<?xml version="1.0"?>
<movies>
    <movie>
        <name>foo</name>
        <released>2020</released>
        <plot>foo is a foofoo</plot>
    </movie>
    <movie>
        <name>bar</name>
        <released>2020</released>
        <plot>bar is a barbar</plot>
    </movie>
</movies>
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