php - access class parent variable not using $this - php

Question

class A {
protected $a = 'aaa';
}
class B extends A {
protected $a = 'bbb';
public function __construct(){
echo parent::$a; // Fatal error: Access to undeclared static property: A::$a in main.php on line 11
}
}
$b = new B();
I want to access $a variable from class A in constructor of class B. Be aware that $a variable is overwritten in class B. How can I access parent::$a?

Answer 1

The only way to do this would be to declare $a as static:
protected static $a = 'aaa';
But that will make the value of parent::$a the same for all instances. If you want separate values, this cannot be done, and you'd be better off renaming the variables, eg one is $a and the other is $b.

Answer 2

class A {
protected $a = 'aaa';
}
class B extends A {
protected $a = 'bbb';
public function __construct(){
echo parent::$a; // Fatal error: ...
}
}
$b = new B();
How can I access parent::$a?
You cant, parent::$a means you are trying to access a static property from a parent class.
instead of doing this,use the constructor to modify $a
class B extends A {
public function __construct(){
// do something with $this->a value here;
}
}
or you'll always overwrite $a if your redeclare it as a property in B.

Answer 3

I just read your comment so I understand your use case a little better now. If you are adding/merging configurations in inheriting classes I'd suggest an alternative approach, adding some behaviour.
As you confirmed above:
class A has a default configuration
class B can optionally pass in config values that can update/add to the default config
In this case, something like this could work for you:
class A
{
protected $config = array(
'foo' => 'foo',
'bar' => 'bar',
'baz' => 'baz',
);
public function __construct(array $config = array())
{
$this->config = array_merge($this->config, $config);
}
public function getConfig()
{
return $this->config;
}
}
class B extends A
{
// implement
}
$b = new B(array(
'foo' => 'OVERWRITTEN',
'new' => 'NEW',
));
print_r($b->getConfig());
Yields:
Array
(
[foo] => OVERWRITTEN
[bar] => bar
[baz] => baz
[new] => NEW
)
You can also overwrite your default config in the same way when using class A directly.
Alternatively, instead of implementing the merge in __construct() you could implement that as a setConfig() method.
Hope this helps :)
EDIT
I just want to add one more thing: if your config is a multidimensional array, you will have to change how you merge arrays. At first glance array_merge_recursive() might seem like the obvious candidate. However:
$old = array(
'foo' => 'foo',
'bar' => 'bar',
'baz' => array(
'baa' => 'baa',
'boo' => 'boo',
),
);
$new = array(
'foo' => 'FOO',
'baz' => array(
'baa' => 'BAA',
),
'new' => 'new'
);
$merge = array_merge_recursive($old, $new);
print_r($merge);
actually yields:
Array
(
[foo] => Array
(
[0] => foo
[1] => FOO
)
[bar] => bar
[baz] => Array
(
[baa] => Array
(
[0] => baa
[1] => BAA
)
[boo] => boo
)
[new] => new
)
Probably not what you are looking for! Instead use array_replace_recursive():
$merge = array_replace_recursive($old, $new);
print_r($merge);
This yields:
Array
(
[foo] => FOO
[bar] => bar
[baz] => Array
(
[baa] => BAA
[boo] => boo
)
[new] => new
)

Answer 4

#Darragh I made it little different because I didn't want to change my constructors:
abstract class A
{
protected $a = array('a' => 1, 'b' => 2);
public function __construct()
{
$this->mixA();
}
protected function a()
{
return array();
}
protected function mixA()
{
foreach ($this->a() as $key => $val) {
$this->a[$key] = $val; // $val can be an array too (in my case it is)
}
}
}
class B extends A
{
protected function a()
{
return array(
'b' => 'new value',
'c' => 'new variable'
);
}
public function dumpA()
{
var_dump($this->a);
}
}
$b = new B();
$b->dumpA();
So now if I want to change my default configs I just overwrite a() method. mixA() method can be expanded as needed.