Private keyword on class attributes

[MerMer]

I had understood that the keyword "private" on a variable within a class, prevents a subclass from accessing that variable.

The code fragment below seems to show that this is not the case.

When the variables in the base class are set to private I had expected the code below to fail because the subclass wouldn't be able to access the variables in the base class.

However, I find that by using "private" the code prevents the subclass from using its own variable but allows access to the base class's variable(s).

Can anyone explain what is going on? Or am I missing something fundamental?

Thanks

MerMer

class Example //base class
{
//class-wide variables
private $var1=2;
private $var2=8;

//function to gather two numbers
function multiply_numbers()
{
    $number3 = $this->var1 * $this->var2;
    return($number3);
}

//function to add numbers together
function add_numbers()
{
    echo $this->var1 + $this->var2;
}        

}

//THIS IS THE SUBCLASS
class Example2 extends Example
{
var $var1=3;
function get()
{echo $this->var1;}
}

$example = new Example;//instatiate base class
$example2 = new Example2; //instatiate subclass
echo $example->multiply_numbers(); //returns 16

echo "<BR/>";
echo $example2->multiply_numbers(); //returns 24 s if variables in the base class are set to public but 16 if set to private!
echo "<BR/>";
echo $example->multiply_numbers(); //shows that var in main class has not been overwritten
echo "<BR/>";

[bpat1434]

Your definition of a private variable is wrong.

Private member variables and functions are not accessible to code outside the class and are used for the class's internal implementation. (Lecky-Thompson, et. al., Professional PHP5. 2005. 27.).

[Shrike]

When you call the following code

echo $example->multiply_numbers(); 

You are calling a method in the parent class (which of course has access to it's own private data). In PHP class methods default to an access of public when none is specified.

Try overriding the method in class Example2 to to get the error you are after :evilgrin:

[ahundiak]

The other thing is that in Example2 you are creating a new $var1. Try adding
error_reporting(E_ALL | E_STRICT);
To your file. I suspect you are getting warnings and just not seeing them.

By the way, bpat is wrong. Private variables are not accessible by sub classes. bpat described protected variables.

[Shrike]

Actually your second example surprised me a little. But it makes sense - if a parent and a child class both define a public variable with the same name, the child class will always in effect override the parent one.

[Weedpacket]

There is also a slight glitch in the comments:

//class-wide variables
private $var1=2;
private $var2=8;

$var1 and $var2 are instance variables, with each Example object having their own copies; they're not available to the class itself.

<?php
class Foo
{
	public $var = 42;
}

$t = new Foo;
echo $t->var;
echo "\n";
echo Foo::$var;

What the comment is describing are static variables, which need to be declared as such.