PHP: call child constructor from static method in parent

Question

I want to have a static method in a parent class that creates instances of whatever subclass i call this method on.

An example to make this more clear:

class parent {
    public static method make_objects($conditions){
        for (...){
            // here i want to create an instance
            // of whatever subclass i am calling make_objects on
            // based on certain $conditions
        }
    }
}

class sub extends parent{
    ...
}

$objects = sub::make_objects($some_conditions);

Answer 1

As of php 5.3 you can use the static keyword for this

<?php
class A {
  public static function newInstance() {
    $rv = new static();  
    return $rv;
  }
}
class B extends A { }
class C extends B { }

$o = A::newInstance(); var_dump($o);
$o = B::newInstance(); var_dump($o);
$o = C::newInstance(); var_dump($o);

prints

object(A)#1 (0) {
}
object(B)#2 (0) {
}
object(C)#1 (0) {
}

edit: another (similar) example

<?php
class A {
  public static function newInstance() {
    $rv = new static();  
    return $rv;
  }

  public function __construct() { echo " A::__construct\n"; }
}
class B extends A {
  public function __construct() { echo " B::__construct\n"; }
}
class C extends B {
  public function __construct() { echo " C::__construct\n"; }   
}

$types = array('A', 'B', 'C');
foreach( $types as $t ) {
  echo 't=', $t, "\n";
  $o = $t::newInstance();
  echo '  type of o=', get_class($o), "\n";
}

prints

t=A
 A::__construct
  type of o=A
t=B
 B::__construct
  type of o=B
t=C
 C::__construct
  type of o=C

Answer 2

I think you want something like this:

class parent {
  public static function make_object($conditionns) {
    if($conditions == "case1") {
      return new sub();
    }
  }
}

class sub extends parent {

}

Now you can create an instance like this:

$instance = parent::make_object("case1");

or

$instance = sub::make_object("case1");

But why would you want all the sub classes to extend the parent? Shouldn't you much rather have a parent for your models (sub classes) and then a factory class, that creates the instances for this models depending on the conditions given?