👍 But the implementation seems too complicated

1. It should be $defaultParameters not $defaultParametr.

2. What about putting in $defaultParametr only parameters with default value and then $item += $defaultParameters; + wrap the array walk with if ($defaultParameters)?

3. And I'm old-fashioned and use foreach instead of array_walk.

foreach ($method->getParameters() as $parameter) {
    if ($parameter->isDefaultValueAvailable()) {
        $defaults[$parameter->getName()] = $parameter->getDefaultValue();
    }
}

if (isset($defaults)) {
    foreach ($parameters as $name => $value) {
        $parameters[$name] += $defaults;
    }
}

Thanks for review!

1. Sure, it was a typo. I'll change it.
2. Parameters must be in same order as method's parameters so we cannot use $item += $defaultParameters;.
3. Me too, but I tried to be new-fashioned :)

 Parameters must be in same order as method parameters

I didn't know that. It that case I would fix the method invocation so that the order would be irrelevant as I would expected from named parameters.

Edit: or maybe not. You would need to reiterate the parameters in method reflection which does not make much sense.

Thanks, that was fast :)