What are the different methods for utilizing functions from one class in another in PHP, specifically in the context of Dependency Injection, Singleton, and Child class?

When utilizing functions from one class in another in PHP, there are several methods such as Dependency Injection, Singleton pattern, and extending the class as a child class. Dependency Injection involves passing an instance of the class as a parameter to the dependent class. Singleton pattern ensures that only one instance of the class exists. Extending a class allows the child class to inherit the functions of the parent class. 

// Dependency Injection
class ClassA {
    public function doSomething() {
        return 'Doing something in Class A';
    }
}

class ClassB {
    private $classA;

    public function __construct(ClassA $classA) {
        $this->classA = $classA;
    }

    public function useClassAFunction() {
        return $this->classA->doSomething();
    }
}

$classA = new ClassA();
$classB = new ClassB($classA);
echo $classB->useClassAFunction();

// Singleton
class Singleton {
    private static $instance;

    public static function getInstance() {
        if (!self::$instance) {
            self::$instance = new self();
        }
        return self::$instance;
    }

    public function doSomething() {
        return 'Doing something in Singleton';
    }
}

$singleton = Singleton::getInstance();
echo $singleton->doSomething();

// Child class
class ParentClass {
    public function doSomething() {
        return 'Doing something in ParentClass';
    }
}

class ChildClass extends ParentClass {
    public function doSomethingElse() {
        return 'Doing something else in ChildClass';
    }
}

$childClass = new ChildClass();
echo $childClass->doSomething();
echo $childClass->doSomethingElse();

Keywords

Dependency Injection Singleton Child class Inheritance Method chaining

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