Using ReflectionClass::newInstanceWithoutConstructor and ReflectionProperty::setValue allow to set a value for a readonly promoted property. For example this code works (tested on PHP 8.2) :
<?php
class Test
{
public function __construct(public readonly string $name)
{}
}
$test1 = new Test('test1');
$reflectionProperty = new ReflectionProperty(Test::class, 'name');
$reflectionProperty->setValue($test1, 'error');
$reflectionClass = new ReflectionClass(Test::class);
$test2 = $reflectionClass->newInstanceWithoutConstructor();
$reflectionProperty->setValue($test2, 'test2');
echo $test2->name;
