Why don't inherited methods use child properties? (PHP)

Question

I'm trying to get the code below to work in child classes. But it keeps failing because it is checking the basicDbClass rather than the child class.

For those complaining and voting my question down because of my use of static methods and variables, the static getBy method serves the purpose of returning an array of matching class objects. If you have a complaint about that methodology, then rather voting down every question that uses code you don't agree with, look for the next question about that subject and then give them your brilliant answer. If you're really antsy, I'll even do you the favor of asking that question so that you can get this out of your system.

The main problem is in static::$data_table. When I do the following, I get a fatal error:

access to undeclared static property: basicDbClass::$data_table

---

$obj_array = childClass::getBy('id',5);


class basicDbClass {

    public static function getBy($property,$value) {
        if(!property_exists(get_called_class(),$property) ) {
            debug::add('errors',__FILE__,__LINE__,__METHOD__,$property . ' is not a valid property.');
            return false;
        }
        $obj_array = array();
        dbConnection::connect(MAIN_DB);
        $sql = sprintf('SELECT `id` FROM `%s` WHERE `%s` = \'%s\'',static::$data_table,mysql_real_escape_string($property),mysql_real_escape_string($value));
        $result = mysql_query($sql);
        if($result === false) {
            return false;
        }
        while($row = mysql_fetch_assoc($result)) {
            $obj_array[$row['id']] = new static($row['id']);
        }
        return $obj_array;
    }
}

class childClass extends basicDbClass {
  protected static $data_table = 'child_class_table';
  protected $name;
  protected $id;
}

Answer 1

First of all I think you might have a typo, childClass doesn't extend basicDbClass.

With that said, as long as you are using PHP 5.3.0 or later, the static keyword should work as described in the documentation for "late static bindings" which provides the polymorphic behavior for static properties that you are looking for (this is weird to me, since I am more accustomed to the inheritance rules in Java).

If you do not define $data_table in the parent class basicDbClass, any call of basicDbClass::getBy on that class directly would fail with a similar error to what you described.

Here's a simple example that shows what should work:

<?php

class A {
    protected static $value = 'From class A';

    public static function output() {
        echo static::$value;
    }
}

class B extends A{
    protected static $value = 'From class B';
}

A::output(); // "From class A"
B::output(); // "From class B"

Answer 2

What version of PHP are you using? This code works without any errors for me using PHP 5.3.13:

childClass::getBy();
// output: child_class_table

class basicDbClass {
  public static function getBy() {
    print_r(static::$data_table);
  }
}

class childClass extends basicDbClass {
  protected static $data_table = 'child_class_table';
}