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Official References (From the online PHP Manual):

• Classes and Objects (PHP 5): http://www.php.net/manual/en/language.oop5.php
• Type Operators: http://www.php.net/manual/en/language.operators.type.php
• Class/Object Functions: http://www.php.net/manual/en/book.classobj.php

Basic

<?php

// No methods in an interface are defined
interface ClassInterface {

  // All methods in an interface must be public
  public function method_normal($arg);
}


// Any class that contains one abstract method must also be abstract
// A class can implement at most one interface
abstract class ClassAbstract implements ClassInterface {

  /* Member Declaration */
  /*--------------------*/

  // Initializer must be a constant expression
  public $var_public = 1;   // Visible publicly
  protected $var_protected; // Visible by inherited classes
  private $var_private;     // Visible within this class
  // Protected/Private members are accessible in different instances
  // of the same class

  // Static members are accessible without instantiation of the class
  // Static members cannot be accessed with an instantiated class object
  // Static members/methods cannot be accessed through -> operator
  public static $var_static = 'static value';

  // A constant do not use $ symbol
  const var_constant = 'constant value';

  /* Constructor */
  /*-------------*/
  public function __construct($arg1, $arg2='default')
  {
    // Called for each newly-created object
    // Prevents direct object creation if declared private
  }

  /* Destructor */
  /*------------*/
  public function __destruct()
  {
    // Called implicitly if all references to this object are removed
  }

  /* Methods */
  /*---------*/

  public function __clone()
  {
    // Called in the new object cloned allowing changing of properties
    $this->var_public++;
  }

  /* Abstract method only declares the method's signature */
  abstract protected function method_one($arg);

  // Member/Method assumed public if not explicitly specified
  function method_normal($arg)
  {
    if (isset($this)) {
      // $this refers to the calling object
    }
    else {
      // Not defined if called statically
    }
    echo __CLASS__;
  }

  public static function method_static()
  {
    // Static methods are callable without instantiation of the class
    // Calling non-static methods statically results in E_STRICT level warning
  }

  final protected function method_protected()
  {
    // Final methods cannot be overridden by inherited class
  }
}


// Final class cannot be inherited
// A class can inherit at most one base class
final class ClassFinal extends ClassAbstract {

  public $var_one = 'my default value';
  // Can redeclare parent's public and protected variables/methods
  protected $var_protected = 'my protected value';

  public function __construct($arg1)
  {
    // Parent constructors are NOT called implicitly
    parent::__construct($arg1);
  }

  public function __destruct()
  {
    // Parent destructors are NOT called implicitly
    parent::__destruct();
  }

  // Define inherited abstract method with the same or less restricted visibility
  public function method_one($arg)
  {
    parent::method_protected();
  }

  // Override the parent's method_normal();
  public function method_normal($arg)
  {
    // Still can access parent's methods and members overridden
    parent::method_one($arg);

    // Miscellaneous
    $a = new self('self');
    $b = new parent('parent');
    echo parent::var_constant."\n";
    echo parent::$var_static."\n";
  }
}


// The special __autoload() function is invoked when you
// try to use a class/interface which has not been defined yet.
function __autoload($class_name)
{
  require_once "include/{$class_name}.php";
}


$object = new ClassFinal('something');  // Create an object
$object->method_one('arg');  // Call a method
$object_two = clone $object;  // Clone an object

if ($object instanceof ClassAbstract) {
  // True if the object is
  // - an instance of a class
  // - an instance of a class that inherits from a parent class
  // - an instance of a class that implements an interface
}

foreach ($object as $member => $value) {
  // Traverse each visible member
  // Can also implement PHP's internal interfaces Iterator
  // or IteratorAggregate to customize the iteration.
}

if ($object == $object_two) {
  // True iff instances of same class, same attributes, and same values.
}
if ($object === $object_two) {
  // True iff refer to the same instance of the same class
}

function demo_type_hinting(ClassFinal $obj)
{
  // Force a function to only accept an object or the specified class
  // NULL is accepted iff it is used as default paramater value
}

Also check the Class/Object Functions.

Advanced

<?php

class ClassAdvanced {

  /* Overloading */

  // PHP's Overloading (whose interpretation is different than most
  // other OO languages) provides means to dynamically create
  // members and methods.
  // Overloading methods interact with inaccessible members/methods,
  // which have not been declared or invisible in the current scope.

  public function __set($name, $value)
  {
    // Triggered by writing data to inaccessible members
  }

  public function __get($name)
  {
    // Triggered by reading data from inaccessible members
    return $value;
  }

  public function __isset($name)
  {
    // Triggered by calling isset() or empty() on inaccessible members
  }

  public function __unset($name)
  {
    // Triggered by calling unset() on inaccessible members
  }

  public function __call($name, $args)
  {
    // Triggered by calling inaccessible methods in object context
  }

  public function __callStatic($name, $args)
  {
    // Triggered by calling inaccessible methods in static context
  }


  /* Serialization */

  public function __sleep()
  {
    // Triggered by caling serialize() prior to serialization
    // Must return an array with member names that should be serialized
    return array();
  }

  public function __wakeup()
  {
    // Triggered by calling unserialize()
  }


  /* String Conversion */

  public function __toString()
  {
    // Returns the string when converted to a string
    return 'string for this object';
  }


  /* __set_state */
  public static function __set_state(array $array)
  {
    // Triggered by calling var_export()
    // $array contains exported properties: array('property' => value,...)
    // Can create parseable code when var_export() is used with objects
  }
}

Other features not covered here:

• Reflection
• Late Static Binding