DIY PHP Dependency Injection in 10 lines

Here’s a use case for you: call a class method with a dynamic array of parameters, while ensuring the typing and presence of certain required parameters which may or may not be present in that array.

class A {}
class B {}

class Foo {
	public static function bar(B $b, A $a = null) {}

call_user_func_array(['Foo', 'bar'], [new B(), new A()]);

For this to work, the array elements must be in the correct order.

// This won't work:
$config = ['a' => new A(), 'b' => new B()];
call_user_func_array(['Foo', 'bar'], $config);

Dynamically invoking a function is useful when working with polymorphism. But how do you do it when your array of arguments might have other data mixed in, or might be in a different order?

This is the very problem that dependency injection containers exist to solve. There are plenty of libraries (and whole frameworks) that do it well, but when the YAGNI principle applies, you can do it in just 10 lines of code with a little help from PHP’s built-in Reflection module.

function inject (array $container, $class, $method) {
	$args = array_map(
		function (ReflectionParameter $arg) {
			return $arg->getName();
		(new ReflectionMethod($class, $method))->getParameters()
	return array_values(compact($args));

// This works:
$config = ['a' => new A(), 'b' => new B()];
$args = inject($config, 'Foo', 'bar');
call_user_func_array(['Foo', 'bar'], $args);


  • Line 2: extract the array elements out as variables in local scope
  • Lines 3-8: inspect the method and get a simple array of parameter names that the method needs, in the correct order
  • Line 9: compile an array of only those variables required to satisfy the method parameters, in the correct order, from the local scope variables created at line 2.

Side note: I really enjoy PHP’s compact function. I’m surprised I don’t see more developers using it.

Written on October 23, 2019