Tight coupling

FooContext.php

interface FooContext {
    public function foo(); // i.e. cache
    public function bar(); // i.e. dbConnection
    public function baz(); // i.e. logger
}

// If we're not doing dependency injection, then you need to handle permutations like below...
// It's easy to see how quickly this gets out of control

StandardFooContext.php

class StandardFooContext implements FooContext 
{
    public function foo()
    {
        return new FileCache(...);
    }
    
    public function bar()
    {
        return new PDOConnection(...);
    }
    
    public function baz()
    {
        return new Monolog(...);
    }
}

StandardFooContextWithMongoDB.php

class StandardFooContextWithMongoDB implements FooContext 
{
    public function foo()
    {
        return new FileCache(...);
    }
    
    public function bar()
    {
        return new MongoDB(...);
    }
    
    public function baz()
    {
        return new Monolog(...);
    }
}

StandardFooContextWithMongoDBAndRedisCache.php

class StandardFooContextWithMongoDBAndRedisCache implements FooContext 
{
    public function foo()
    {
        return new Redis(...);
    }
    
    public function bar()
    {
        return new MongoDB(...);
    }
    
    public function baz()
    {
        return new Monolog(...);
    }
}