<?xml version="1.0" encoding="utf-8"?>
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 <sect1 xml:id="language.oop5.basic" xmlns="http://docbook.org/ns/docbook">
  <title>The Basics</title>

  <sect2 xml:id="language.oop5.basic.class">
   <title>class</title>
   <para>
    Basic class definitions begin with the
    keyword <literal>class</literal>, followed by a class name,
    followed by a pair of curly braces which enclose the definitions
    of the properties and methods belonging to the class.
   </para>
   <para>
    The class name can be any valid label which is a not a
    PHP <link linkend="reserved">reserved word</link>. A valid class
    name starts with a letter or underscore, followed by any number of
    letters, numbers, or underscores. As a regular expression, it
    would be expressed thus:
    <literal>[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*</literal>.
   </para>
   <para>
    A class may contain its
    own <link linkend="language.oop5.constants">constants</link>, <link linkend="language.oop5.properties">variables</link>
    (called "properties"), and functions (called "methods").
   </para>
   <example>
    <title>Simple Class definition</title>
    <programlisting role="php">
<![CDATA[
<?php
class SimpleClass
{
    // property declaration
    public $var = 'a default value';

    // method declaration
    public function displayVar() {
        echo $this->var;
    }
}
?>
]]>
    </programlisting>
   </example>
   <para>
    The pseudo-variable <varname>$this</varname> is available when a
    method is called from within an object
    context. <varname>$this</varname> is a reference to the calling
    object (usually the object to which the method belongs, but
    possibly another object, if the method is called
    <link linkend="language.oop5.static">statically</link> from the context
    of a secondary object).
   </para>
   <para>
    <example>
     <title>Some examples of the <varname>$this</varname> pseudo-variable</title>
     <programlisting role="php">
<![CDATA[
<?php
class A
{
    function foo()
    {
        if (isset($this)) {
            echo '$this is defined (';
            echo get_class($this);
            echo ")\n";
        } else {
            echo "\$this is not defined.\n";
        }
    }
}

class B
{
    function bar()
    {
        // Note: the next line will issue a warning if E_STRICT is enabled.
        A::foo();
    }
}

$a = new A();
$a->foo();

// Note: the next line will issue a warning if E_STRICT is enabled.
A::foo();
$b = new B();
$b->bar();

// Note: the next line will issue a warning if E_STRICT is enabled.
B::bar();
?>
]]>
     </programlisting>
     &example.outputs;
     <screen>
<![CDATA[
$this is defined (A)
$this is not defined.
$this is defined (B)
$this is not defined.
]]>
     </screen>
    </example>
   </para>
  </sect2>

  <sect2 xml:id="language.oop5.basic.new">
   <title>new</title>
   <para>
    To create an instance of a class, a new object must be created and
    assigned to a variable.  An object will always be assigned when
    creating a new object unless the object has a
    <link linkend="language.oop5.decon">constructor</link> defined that throws an
    <link linkend="language.exceptions">exception</link> on error. Classes
    should be defined before instantiation (and in some cases this is a
    requirement).
   </para>
   <example>
    <title>Creating an instance</title>
    <programlisting role="php">
<![CDATA[
<?php
$instance = new SimpleClass();

// This can also be done with a variable:
$className = 'Foo';
$instance = new $className(); // Foo()
?>
]]>
    </programlisting>
   </example>
   <para>
    In the class context, it is possible to create a new object by
    <literal>new self</literal> and <literal>new parent</literal>.
   </para>
   <para>
    When assigning an already created instance of a class to a new variable, the new variable
    will access the same instance as the object that was assigned. This
    behaviour is the same when passing instances to a function. A copy
    of an already created object can be made by
    <link linkend="language.oop5.cloning">cloning</link> it.
   </para>
   <example>
    <title>Object Assignment</title>
    <programlisting role="php">
<![CDATA[
<?php

$instance = new SimpleClass();

$assigned   =  $instance;
$reference  =& $instance;

$instance->var = '$assigned will have this value';

$instance = null; // $instance and $reference become null

var_dump($instance);
var_dump($reference);
var_dump($assigned);
?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
NULL
NULL
object(SimpleClass)#1 (1) {
   ["var"]=>
     string(30) "$assigned will have this value"
}
]]>
    </screen>
   </example>
  </sect2>

  <sect2 xml:id="language.oop5.basic.extends">
   <title>extends</title>
   <para>
    A class can inherit the methods and properties of another class by
    using the keyword <literal>extends</literal> in the class
    declaration. It is not possible to extend multiple classes; a
    class can only inherit from one base class.
   </para>
   <para>
    The inherited methods and properties can be overridden by
    redeclaring them with the same name defined in the parent
    class. However, if the parent class has defined a method
    as <link linkend="language.oop5.final">final</link>, that method
    may not be overridden.  It is possible to access the overridden
    methods or static properties by referencing them
    with <link linkend="language.oop5.paamayim-nekudotayim">parent::</link>.
   </para>
   <para>
    When overriding methods, the parameter signature should remain the same or
    PHP will generate an <constant>E_STRICT</constant> level error. This does
    not apply to the constructor, which allows overriding with different
    parameters.
   </para>
   <example>
    <title>Simple Class Inheritance</title>
    <programlisting role="php">
<![CDATA[
<?php
class ExtendClass extends SimpleClass
{
    // Redefine the parent method
    function displayVar()
    {
        echo "Extending class\n";
        parent::displayVar();
    }
}

$extended = new ExtendClass();
$extended->displayVar();
?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
Extending class
a default value
]]>
    </screen>
   </example>
  </sect2>

 </sect1>

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