CustomNormalizerForEmbeddedArray.php

<?php
// Simple hydrator
final class Hydrator
{
    public function hydrate($object, array $data = [])
    {
        foreach ($data as $property => $value) {
            $method = 'set'.ucfirst($property);
            if (\method_exists($object, $method)) {
                $object->{$method}($value);
            }
        }
    }
}

class Root {
 /**
 * @var string
 */
 $firstName;
  
 /**
 * @var string
 */
 $lastName; //string
  
  // other properties
 
  /**
   * @var FooEmbedded
   */
 $fooEmbedded;
  
  /**
   * @var BarEmbedded
   */
 $barEmbedded;
  
   /**
   * @var DummyEmbedded
   */
 $dummyEmbedded;
  
  // setters
}

// index.php

$data = [
  'firstname' => 'Smaone',
  'lastName' => 'LesZommes',
  // nearly 20 properties in the root
  'dummyEmbedded' => ['property1'=> 'value', 'property2'=> 'value',],//...
  'barEmbedded' => ['property1'=> 'value', 'property2'=> 'value',]//...
  'fooEmbedded' => ['property1'=> 'value', 'property2'=> 'value',]//...
  ];

$embeddedRelations = [
  'dummyEmbedded'=> FooEmbedded::class,
 'barEmbedded' => BarEmbedded,
 'fooEmbedded'=> FooEmbedded::class
 ];

// Replace the emmbedded array by an object 
foreach ($embeddedRelations as $relationName => $fqcn) {
            $relationObject = new $fqcn;
            (new Hydrator)->hydrate($relationObject, $data[$relationName]);
            $data[$relationName] = $relationObject;
        }

// Hydrate the Root :-)
$root = new Root;
(new Hydrator)->hydrate($root, $data);