An example of how dependency injection works

injection.php

<?php

// create our service container
$serviceContainer = [
    TestClass::class => new TestClass('Matt')
];

// create our test class
class TestClass {

    public function __construct(string $name)
    {
        $this->name = $name;
    }

    public function welcome(): string
    {
        return sprintf('Hey there %s', $this->name);
    }

}

// create our class that uses dependency injection
class Controller {
    public function index(TestClass $class)
    {
        echo 'Hey there ' . $class->name;
    }
}

// create our injection method. This will work out anything our
// classes need to run
$run = function ($class, $method) use ($serviceContainer) {

    // build an array of items to inject
    $inject = [];

    // inspect our class using reflection, find the method
    // we're looking for and get the parameters we are looking for
    $reflection = new ReflectionClass($class);
    $reflectionMethod = $reflection->getMethod($method);
    $reflectionParameters = $reflectionMethod->getParameters();

    // loop through each parameter and see if we can get it from our service container.
    foreach ($reflectionParameters as $parameter) {

        // if we have a class that we're looking for within our service container,
        // add it to our array of injected parameters
        if (array_key_exists($parameter->getType()->getName(), $serviceContainer)) {
            $inject[$parameter->getName()] = $serviceContainer[$parameter->getType()->getName()];
        }
    }

    // create our class and run our method with our new parameters
    return (new $class)->{$method}(...$inject);
};

// call our run method
$run(Controller::class, 'index');