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> Often you need classes with similar variables and functions
to another existing class. In fact, it is good practice to
define a generic class which can be used in all your
projects and adapt this class for the needs of each of your
specific projects. To facilitate this, classes can be
extensions of other classes. The extended or derived class
has all variables and functions of the base class (this is
called 'inheritance' despite the fact that nobody died) and what
you add in the extended definition. It is not possible to
subtract from a class, that is, to undefine any existing
functions or variables. An extended class is always dependent
on a single base class, that is, multiple inheritance is
not supported. Classes are extended using the keyword 'extends'.
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><code><font color="#000000">
<font color="#0000BB"><?php<br /></font><font color="#007700">class </font><font color="#0000BB">Named_Cart </font><font color="#007700">extends </font><font color="#0000BB">Cart </font><font color="#007700">{<br /> var </font><font color="#0000BB">$owner</font><font color="#007700">;<br /> <br /> function </font><font color="#0000BB">set_owner </font><font color="#007700">(</font><font color="#0000BB">$name</font><font color="#007700">) {<br /> </font><font color="#0000BB">$this</font><font color="#007700">-></font><font color="#0000BB">owner </font><font color="#007700">= </font><font color="#0000BB">$name</font><font color="#007700">;<br /> }<br />}<br /></font><font color="#0000BB">?></font>
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> This defines a class Named_Cart that has all variables and functions of
Cart plus an additional variable <VAR
CLASS="varname"
>$owner</VAR
> and an
additional function set_owner(). You create a named cart the usual way and
can now set and get the carts owner. You can still use normal cart
functions on named carts:
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<font color="#0000BB"><?php<br />$ncart </font><font color="#007700">= new </font><font color="#0000BB">Named_Cart</font><font color="#007700">; </font><font color="#FF8000">// Create a named cart<br /></font><font color="#0000BB">$ncart</font><font color="#007700">-></font><font color="#0000BB">set_owner</font><font color="#007700">(</font><font color="#DD0000">"kris"</font><font color="#007700">); </font><font color="#FF8000">// Name that cart<br /></font><font color="#007700">print </font><font color="#0000BB">$ncart</font><font color="#007700">-></font><font color="#0000BB">owner</font><font color="#007700">; </font><font color="#FF8000">// print the cart owners name<br /></font><font color="#0000BB">$ncart</font><font color="#007700">-></font><font color="#0000BB">add_item</font><font color="#007700">(</font><font color="#DD0000">"10"</font><font color="#007700">, </font><font color="#0000BB">1</font><font color="#007700">); </font><font color="#FF8000">// (inherited functionality from cart)<br /></font><font color="#0000BB">?></font>
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><P
> This is also called a "parent-child" relationship. You create a class,
parent, and use <VAR
CLASS="literal"
>extends</VAR
> to create a new class
<SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>based</I
></SPAN
> on the parent class: the child class. You can
even use this new child class and create another class based on this child
class.
</P
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><B
>Opmerking: </B
>
Classes must be defined before they are used! If you want the class
<VAR
CLASS="literal"
>Named_Cart</VAR
> to extend the class
<VAR
CLASS="literal"
>Cart</VAR
>, you will have to define the class
<VAR
CLASS="literal"
>Cart</VAR
> first. If you want to create another class called
<VAR
CLASS="literal"
>Yellow_named_cart</VAR
> based on the class
<VAR
CLASS="literal"
>Named_Cart</VAR
> you have to define
<VAR
CLASS="literal"
>Named_Cart</VAR
> first. To make it short: the order in which
the classes are defined is important.
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