Hack to properly instantiate class with private constructor:

<?php

class TestClass
{
    private $property;
    private function __construct($argument)
    {
        $this->property = $argument;
    }
}

$ref = new ReflectionClass(TestClass::class);
$instance = $ref->newInstanceWithoutConstructor();
var_dump($instance);
echo PHP_EOL . '------------------------' . PHP_EOL . PHP_EOL;
$constructor = $ref->getConstructor();
$constructor->setAccessible(true);
$constructor->invokeArgs($instance, ['It works!']);
var_dump($instance);

// Output:
// class TestClass#3 (1) {
//   private $property =>
//   NULL
// }
//
// ------------------------
//
// class TestClass#3 (1) {
//   private $property =>
//   string(9) "It works!"
// }

?>

I use reflection class and also detect whether arguments are passed by reference or passed by value
and then initiate/call the method successfully with those arguments:

<?php
    if (count($args) > 1)
    {
        if (method_exists($class_name,  '__construct') === false)
        {
            exit("Constructor for the class <strong>$class_name</strong> does not exist, you should not pass arguments to the constructor of this class!");
        }
    
        $refMethod = new ReflectionMethod($class_name,  '__construct');
        $params = $refMethod->getParameters();
    
        $re_args = array();
    
        foreach($params as $key => $param)
        {
            if ($param->isPassedByReference())
            {
                $re_args[$key] = &$args[$key];
            }
            else
            {
                $re_args[$key] = $args[$key];
            }
        }
    
        $refClass = new ReflectionClass($class_name);
        $class_instance = $refClass->newInstanceArgs((array) $re_args);
    }
?>

the newInstanceArgs function cannot call a class' constructor if it has references in its arguments, so be careful what you pass into it:

<?php
class Foo {
    function __construct (&$arr) {
        $this->arr = &$arr;
    }
    function createInstance () {
        $reflectionClass = new ReflectionClass("Bar");
        
        return $reflectionClass->newInstanceArgs(array($this, $this->arr));
    }
    function mod($key, $val) {
        $this->arr[$key] = $val;
    }
}

class Bar {
    function __construct (&$foo, &$arr) {
        $this->foo = &$foo;
        $this->arr = &$arr;
    }
    function mod($key, $val) {
        $this->arr[$key] = $val;
    }
}

$arr = array();

$foo = new Foo($arr);

$arr["x"] = 1;

$foo->mod("y", 2);

$bar = $foo->createInstance();

$bar->mod("z", 3);

echo "<pre>";
print_r($arr);
print_r($foo);
print_r($bar);
echo "</pre>";

/*
Output:
Warning: Invocation of Bar's constructor failed in [code path] on line 31

Fatal error: Call to a member function mod() on a non-object in [code path] on line 58
*/
?>

This is the way I dynamically instantiate objects in my lightweight IoC container

<?php

class SimpleContainer {

  // ... 

  // Creates an instance of an object with the provided array of arguments
  protected function instantiate($name, $args=array()){
    if(empty($args))
      return new $name();                                                                                                                                                          
    else {
      $ref = new ReflectionClass($name);
      return $ref->newInstanceArgs($args);
    }
  }
  // ... 
}
?>

I explicitly do NOT handle the case where a user passes constructor arguments for a constructor-less class, as I this SHOULD fail.

With PHP 5.6, we can use the ... (T_ELLIPSIS) operator

<?php

class Test {
    public function __construct($a, $b) {
        echo $a . ' ' . $b; 
    }
}

$args = array(12, 34);
new Test(... $args); // Displays "12 34"

?>

Annoyingly, this will throw an exception for classes with no constructor even if you pass an empty array for the arguments. For generic programming you should avoid this function and use call_user_func_array with newInstance.

I misunderstood this function to be a sort of setter of Reflection::newInstance() arguments in an array form rather than a creator of new instances itself.

This function is equivilant to call_user_func_array() while Reflection::newInstance() is equivilant to call_user_func()

Be aware that calling the method newInstanceArgs with an empty array will still call the constructor with no arguments. If the class has no constructor then it will generate an exception.

You need to check if a constructor exists before calling this method or use try and catch to act on the exception.