<?php

class Test
{
    public function __construct($required, $notRequired = "abc")
    {

    }
}

$reflectionClass = new ReflectionClass(Test::class);
$constructor = $reflectionClass->getMethod('__construct');
$parameters = $constructor->getParameters();
foreach ($parameters as $parameter) {
    $name = $parameter->getName();
    $parameter = $parameter->isOptional();
}