An example missing from the documentation is that `ReflectionClass::isInstantiable` will also return false for traits, as well as interfaces and abstract classes.

<?php
trait t {
    // Optional trait methods and properties etc.
}

$reflectionClass = new ReflectionClass("t");
var_dump($reflectionClass->isInstantiable()); // bool(false)
?>

As for classes with private constructors, it is still possible to create an instance by either bypassing the constructor using `ReflectionClass::newInstanceWithoutConstructor`, or by ensuring the class has a method which can create a new instance.

<?php
class p {
    private function __construct() {
        // Optional constructor logic - not called when ReflectionClass::newInstanceWithoutConstructor is used.
    }

    public static function create() {
        return new p;
    }

    // Optional methods and properties etc.
}

// Class is not classed as instantiable.
$reflectionClass = new ReflectionClass("p");
var_dump($reflectionClass->isInstantiable()); // bool(false)

// We're still able to create an instance using one of the two methods.
$p = p::create();
$p = $reflectionClass->newInstanceWithoutConstructor();
?>

The same is also true for protected constructors, however, the class can be instantiated from either parent or child methods, depending on where the constructor is defined.

<?php
class p {
    protected function __construct() {
        // Optional constructor logic.
    }

    public static function create( $class = "" ) {
        if (!$class) {
            $class = get_called_class();
        }
        return new $class;
    }

    // Optional parent methods and properties etc.
}

class c extends p
{
    // Optional child methods and properties etc.
}

// Both child and parent static methods have access to each other's protected constructor.
$p = c::create("p");
$c = p::create("c");

// Both are still not classed as being instantiable.
$reflectionClassP = new ReflectionClass("p");
$reflectionClassC = new ReflectionClass("c");
var_dump($reflectionClassP->isInstantiable()); // bool(false)
var_dump($reflectionClassC->isInstantiable()); // bool(false)

// We're still able to bypass the constructor and create an instance for each.
$p = $reflectionClassP->newInstanceWithoutConstructor();
$c = $reflectionClassC->newInstanceWithoutConstructor();
?>