Difference between private and public visibility PHP keywords (illustrative example)

private_public_test.php

<?php

class A {
  private $foo="a";
  protected $bar="a";
  function getAstuff() {
    return $this->foo.'/'.$this->bar;
  }
}

class B extends A {
  private $foo="b";
  protected $bar="b";
  function getBstuff() {
    return $this->foo.'/'.$this->bar;
  }
}

$b=new B();
echo 'getBstuff: '.$b->getBstuff()."\ngetAstuff: ".$b->getAstuff()."\n";

/*
Output:
getBstuff: b/b
getAstuff: a/b

As you see, methods defined in class A use the private vars of class A
even when used on an instance of child class B. 
Protected attributes however are overridden just like private ones. 
Notice that I do use overridden properties here. 
You could not access $foo from B if it was not overwritten there. 

*/

private_public_test_methods.php

<?php

class A {
  private function foo() {
    return "a";
  }
  protected function bar() {
    return "a";
  }
  function getAstuff() {
    return $this->foo().'/'.$this->bar();
  }
}

class B extends A {
  private function foo() {
    return "b";
  }
  protected function bar() {
    return "b";
  }
  function getBstuff() {
    return $this->foo().'/'.$this->bar();
  }
}

$b=new B();
echo 'getBstuff: '.$b->getBstuff()."\ngetAstuff: ".$b->getAstuff()."\n";


/*
Output:
getBstuff: b/b
getAstuff: a/b

It's just the same thing with methods. 
If I would not override foo() here it would throw an error:
"Fatal error: Call to private method A::foo() from context 'B' in /home/burny/privtest.php on line 23"

*/

private_public_test_static.php

<?php

abstract class A {
  private static $foo="a";
  protected static $bar="a";
  function getAstuff() {
    return self::$foo.'/'.self::$bar;
  }
  function getAstuffLateStatic() {
    if(get_called_class()=="A") {
      return static::$foo.'/'.static::$bar;
    } else {
      return /*static::$foo*/'X'.'/'.static::$bar;
    }
  }
}

class B extends A {
  private static $foo="b";
  protected static $bar="b";
  function getBstuff() {
    return self::$foo.'/'.self::$bar;
  }
  function getBstuffParent() {
    return /*parent::$foo*/'X'.'/'.parent::$bar;
  }
}

$b=new B();
echo 'B::getBstuff: '.B::getBstuff().
"\nB::getAstuff:    ".B::getAstuff().
"\nB::getBstuffParent:     ".B::getBstuffParent().
"\nB::getAstuffLateStatic: ".B::getAstuffLateStatic().
"\nA::getAstuffLateStatic: ".A::getAstuffLateStatic().
"\n";

/*
Output:

B::getBstuff: b/b
B::getAstuff:    a/a
B::getBstuffParent:     X/a
B::getAstuffLateStatic: X/b
A::getAstuffLateStatic: a/a



Things work similar in static contexts. 
The difference is we can freely choose
between attributes of parent and child class
using parent, static and self. 

When using self, we do always stay in the defining
class and we can access our own private attributes.

When using parent and static, we can only access
protected attributes, accessing the child's class
private properties throws this error:

PHP Fatal error:  Cannot access private property B::$foo in /home/amoebe/Documents/854668/private_public_test_static.php on line 13

And accessing the parent's private stuff this one:

PHP Fatal error:  Cannot access private property A::$foo in /home/amoebe/Documents/854668/private_public_test_static.php on line 25

Hence private locks access in both directions. 

*/