php-doc-en/language/oop5/visibility.xml

<?xml version="1.0" encoding="utf-8"?>
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 <sect1 xml:id="language.oop5.visibility" xmlns="http://docbook.org/ns/docbook">
  <title>Visibility</title>
  <para>
   The visibility of a property or method can be defined by prefixing
   the declaration with the keywords <emphasis>public</emphasis>,
   <emphasis>protected</emphasis> or
   <emphasis>private</emphasis>. Class members declared public can be
   accessed everywhere. Members declared protected can be accessed
   only within the class itself and by inherited classes. Members
   declared as private may only be accessed by the class that defines
   the member.
  </para>

  <sect2 xml:id="language.oop5.visibility-members">
   <title>Property Visibility</title>
   <para>
    Class properties must be defined as public, private, or
    protected. If declared using <emphasis>var</emphasis>,
    the property will be defined as public.
   </para>
   <para>
    <example>
     <title>Property declaration</title>
     <programlisting role="php">
<![CDATA[
<?php
/**
 * Define MyClass
 */
class MyClass
{
    public $public = 'Public';
    protected $protected = 'Protected';
    private $private = 'Private';

    function printHello()
    {
        echo $this->public;
        echo $this->protected;
        echo $this->private;
    }
}

$obj = new MyClass();
echo $obj->public; // Works
echo $obj->protected; // Fatal Error
echo $obj->private; // Fatal Error
$obj->printHello(); // Shows Public, Protected and Private


/**
 * Define MyClass2
 */
class MyClass2 extends MyClass
{
    // We can redeclare the public and protected method, but not private
    protected $protected = 'Protected2';

    function printHello()
    {
        echo $this->public;
        echo $this->protected;
        echo $this->private;
    }
}

$obj2 = new MyClass2();
echo $obj2->public; // Works
echo $obj2->protected; // Fatal Error
echo $obj2->private; // Undefined
$obj2->printHello(); // Shows Public, Protected2, Undefined

?> 
]]>
     </programlisting>
    </example>
   </para>
   <note>
    <simpara>
     The PHP 4 method of declaring a variable with the
     <emphasis>var</emphasis> keyword is still supported for compatibility
     reasons (as a synonym for the public keyword). In PHP 5 before 5.1.3, its
     usage would generate an <constant>E_STRICT</constant> warning.
    </simpara>
   </note>
  </sect2>

  <sect2 xml:id="language.oop5.visiblity-methods">
   <title>Method Visibility</title>
   <para>
    Class methods may be defined as public, private, or
    protected. Methods declared without any explicit visibility
    keyword are defined as public.
   </para>
   <para>
    <example>
     <title>Method Declaration</title>
     <programlisting role="php">
<![CDATA[
<?php
/**
 * Define MyClass
 */
class MyClass
{
    // Declare a public constructor
    public function __construct() { }

    // Declare a public method
    public function MyPublic() { }

    // Declare a protected method
    protected function MyProtected() { }

    // Declare a private method
    private function MyPrivate() { }

    // This is public
    function Foo()
    {
        $this->MyPublic();
        $this->MyProtected();
        $this->MyPrivate();
    }
}

$myclass = new MyClass;
$myclass->MyPublic(); // Works
$myclass->MyProtected(); // Fatal Error
$myclass->MyPrivate(); // Fatal Error
$myclass->Foo(); // Public, Protected and Private work


/**
 * Define MyClass2
 */
class MyClass2 extends MyClass
{
    // This is public
    function Foo2()
    {
        $this->MyPublic();
        $this->MyProtected();
        $this->MyPrivate(); // Fatal Error
    }
}

$myclass2 = new MyClass2;
$myclass2->MyPublic(); // Works
$myclass2->Foo2(); // Public and Protected work, not Private

class Bar 
{
    public function test() {
        $this->testPrivate();
        $this->testPublic();
    }

    public function testPublic() {
        echo "Bar::testPublic\n";
    }
    
    private function testPrivate() {
        echo "Bar::testPrivate\n";
    }
}

class Foo extends Bar 
{
    public function testPublic() {
        echo "Foo::testPublic\n";
    }
    
    private function testPrivate() {
        echo "Foo::testPrivate\n";
    }
}

$myFoo = new Foo();
$myFoo->test(); // Bar::testPrivate 
                // Foo::testPublic
?>
]]>
     </programlisting>
    </example>
   </para>
  </sect2>

  <sect2 xml:id="language.oop5.visibility-other-objects">
   <title>Visibility from other objects</title>
   <para>
    Objects of the same type will have access to each others private and
    protected members even though they are not the same instances. This is
    because the implementation specific details are already known when inside
    those objects.
   </para>
   <example>
    <title>Accessing private members of the same object type</title>
    <programlisting role="php">
<![CDATA[
<?php
class Test
{
    private $foo;

    public function __construct($foo)
    {
        $this->foo = $foo;
    }

    private function bar()
    {
        echo 'Accessed the private method.';
    }

    public function baz(Test $other)
    {
        // We can change the private property:
        $other->foo = 'hello';
        var_dump($other->foo);

        // We can also call the private method:
        $other->bar();
    }
}

$test = new Test('test');

$test->baz(new Test('other'));
?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
string(5) "hello"
Accessed the private method.
]]>
    </screen>
   </example>
  </sect2>
 </sect1>
 
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