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6.4. Declaring a Class

To design your program or code library in an object-oriented fashion, you'll need to define your own classes, using the class keyword. A class definition includes the class name and the properties and methods of the class. Class names are case-insensitive and must conform to the rules for PHP identifiers. The class name stdClass is reserved. Here's the syntax for a class definition:

class classname [ extends baseclass ]
{
    [ var $property [ = value ]; ... ]

    [ function functionname (args) {
          // code 
      }
      ...
    ]
}

6.4.1. Declaring Methods

A method is a function defined inside a class. Although PHP imposes no special restrictions, most methods act only on data within the object in which the method resides. Method names beginning with two underscores (__) may be used in the future by PHP (and are currently used for the object serialization methods _ _sleep( ) and _ _wakeup( ), described later in this chapter), so it's recommended that you do not begin your method names with this sequence.

Within a method, the $this variable contains a reference to the object on which the method was called. For instance, if you call $rasmus->birthday( ), inside the birthday( ) method, $this holds the same value as $rasmus. Methods use the $this variable to access the properties of the current object and to call other methods on that object.

Here's a simple class definition of the Person class that shows the $this variable in action:

class Person {
    var $name;

    function get_name ( ) {
        return $this->name;
    }

    function set_name ($new_name) {
        $this->name = $new_name;
    }
}

As you can see, the get_name( ) and set_name( ) methods use $this to access and set the $name property of the current object.

There are no keywords or special syntax for declaring a static method. A static method simply doesn't use $this, because the method is called on a class and not on an object. For example:

class HTML_Stuff {
  function start_table( ) {
    echo "<table border='1'>\n";
  }
  function end_table ( ) {
    echo "</table>\n";
  }
}
HTML_Stuff->start_table( );
// print HTML table rows and columns
HTML_Stuff->end_table( );

6.4.5. References

When you assign an object to another variable, you create a copy:

$fred = new Person;
$copy = $fred;
$fred->name("Fred");
print $copy->name();      // does not print "Fred"

You now have two Person objects, $fred and $copy, with independent property values. This is also the case when you assign the results of a call to a constructor, as shown here:

$fred = new Person;

The object created by the Person constructor is copied, and the copy is stored in $fred. This means that $this in the constructor and $fred actually refer to two different objects. If the constructor creates an alias to $this through a reference, it won't create an alias to $fred. For example:

$people = array();
class Person {
    function Person () {
      global $people;
      $people[] =& $this;
    }
}
$fred = new Person;
$fred->name = "Fred";
$barney =& new Person;
$barney->name = "Barney";
var_dump($people);
array(2) {
   [0]=>
   &object(person)(0) {
   }
   [1]=>
   &object(person)(1) {
     ["name"]=>
     string(6) "Barney"
   }
}

$fred is a copy of the object that the constructor stored in $people[0], while $barney is an alias for the object that the constructor stored in $people[1]. When we change the properties of $fred, we're not changing the object that is in $people[0]. However, when we change the properties of $barney, we are changing the object in $people[1].

To prevent copying on assignment, assign by reference:

$obj =& new Class;

This code makes $obj an alias for the new object, which was $this in the constructor. If the constructor stores a reference to $this, it keeps a reference to $obj.

The documentation for a class should say whether you need to use =& with its constructor. In most cases, this isn't necessary.



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