PHP: How to Pass child class __construct() arguments to parent::__construct()?

Question

I have a class in PHP like so:

class ParentClass {
    function __construct($arg) {
        // Initialize a/some variable(s) based on $arg
    }
}


        

Answer 1

My way of doing it (tested in PHP 7.1):

class ParentClass {
    public function __construct(...$args) {
        print 'Parent: ' . count($args) . PHP_EOL;
    }
}

class ChildClass extends ParentClass {
    public function __construct(...$args) {
        parent::__construct(...$args);
        print 'Child: ' . count($args). PHP_EOL;
    }
}

$child = new ChildClass(1, 2, 3, new stdClass);

//Output: 
//Parent: 4
//Child: 4

Test it https://3v4l.org/csJ68

In this case, the parent and the child have the same constructor signature. It also works if you want to unpack your arguments in the parent constructor:

class ParentClass {
    public function __construct($a, $b, $c, $d = null) {
        print 'Parent: ' . $a .  $b .  $c . PHP_EOL;
    }
}

class ChildClass extends ParentClass {
    public function __construct(...$args) {
        parent::__construct(...$args);
    }
}

$child = new ChildClass(1, 2, 3);

//Output: Parent: 123

Test it https://3v4l.org/JGE1A

It is also worth noticing that in PHP >= 5.6 you can enforce types for variadic functions (scalar types in PHP >= 7):

class ParentClass {
    public function __construct(DateTime ...$args) {
        //Do something
    }
}