node-enc.php

<?php
class Node implements JsonSerializable {
	public function __construct( $name, $contents ) {
		$this->name = $name;
		$this->store = [];
		foreach ( (array)$contents as $k => $v ) {
			if ( is_int( $k ) ) {
				$this->addChild( $v );
			} else {
				$this->addAttribute( $k, $v );
			}
		}
	}

	public function addChild( $child ) {
		$this->store[] = $child;
	}

	public function addAttribute( $attribute, $value ) {
		$this->addChild( new Node( "@{$attribute}", $value ) );
	}

	public function jsonSerialize() {
		return [ $this->name, $this->store ];
	}
}



$k = 42;
$val = 'abcd';

$fancyNode = new Node( 'part', [
	new Node( 'name', [ 'index' => $k ] ),
	new Node( 'value', $val ),
] );

$rawNode = [ 'part', [
	[ 'name', [ [ '@index', [ $k ] ] ] ],
	[ 'value', [ strval( $val ) ] ],
] ];

assert( json_encode( $fancyNode ) === json_encode( $rawNode ) );