archived-doc-en/language/oop5/abstract.xml

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 <sect1 xml:id="language.oop5.abstract" xmlns="http://docbook.org/ns/docbook">
  <title>Class Abstraction</title>

  <para>
   PHP has abstract classes, methods, and properties.
   Classes defined as abstract cannot be instantiated, and any class that
   contains at least one abstract method or property must also be abstract.
   Methods defined as abstract simply declare the method's signature and whether it is public or protected;
   they cannot define the implementation. Properties defined as abstract
   may declare a requirement for <literal>get</literal> or <literal>set</literal>
   behavior, and may provide an implementation for one, but not both, operations.
  </para>

  <para>
   When inheriting from an abstract class, all methods marked abstract in
   the parent's class declaration must be defined by the child class,
   and follow the usual
   <link linkend="language.oop5.inheritance">inheritance</link> and
   <link linkend="language.oop.lsp">signature compatibility</link> rules.
  </para>

  <simpara>
   As of PHP 8.4, an abstract class may declare an abstract property, either public or protected.
   A protected abstract property may be satisfied by a property that is readable/writeable from either
   protected or public scope.
  </simpara>
  <simpara>
   An abstract property may be satisfied either by a standard property or by a property
   with defined <link linkend="language.oop5.property-hooks">hooks</link>, corresponding to the required operation.
  </simpara>

  <example>
   <title>Abstract method example</title>
   <programlisting role="php">
<![CDATA[
<?php

abstract class AbstractClass
{
    // Force extending class to define this method
    abstract protected function getValue();
    abstract protected function prefixValue($prefix);

    // Common method
    public function printOut()
    {
        print $this->getValue() . "\n";
    }
}

class ConcreteClass1 extends AbstractClass
{
    protected function getValue()
    {
        return "ConcreteClass1";
    }

    public function prefixValue($prefix)
    {
        return "{$prefix}ConcreteClass1";
    }
}

class ConcreteClass2 extends AbstractClass
{
    public function getValue()
    {
        return "ConcreteClass2";
    }

    public function prefixValue($prefix)
    {
        return "{$prefix}ConcreteClass2";
    }
}

$class1 = new ConcreteClass1();
$class1->printOut();
echo $class1->prefixValue('FOO_'), "\n";

$class2 = new ConcreteClass2();
$class2->printOut();
echo $class2->prefixValue('FOO_'), "\n";

?>
]]>
   </programlisting>
   &example.outputs;
   <screen>
<![CDATA[
ConcreteClass1
FOO_ConcreteClass1
ConcreteClass2
FOO_ConcreteClass2
]]>
   </screen>
  </example>

  <example>
   <title>Abstract method example</title>
    <programlisting role="php">
<![CDATA[
<?php

abstract class AbstractClass
{
    // An abstract method only needs to define the required arguments
    abstract protected function prefixName($name);
}

class ConcreteClass extends AbstractClass
{
    // A child class may define optional parameters which are not present in the parent's signature
    public function prefixName($name, $separator = ".")
    {
        if ($name == "Pacman") {
            $prefix = "Mr";
        } elseif ($name == "Pacwoman") {
            $prefix = "Mrs";
        } else {
            $prefix = "";
        }

        return "{$prefix}{$separator} {$name}";
    }
}

$class = new ConcreteClass();
echo $class->prefixName("Pacman"), "\n";
echo $class->prefixName("Pacwoman"), "\n";

?>
]]>
   </programlisting>
   &example.outputs;
   <screen>
<![CDATA[
Mr. Pacman
Mrs. Pacwoman
]]>
   </screen>
  </example>
  <example>
   <title>Abstract property example</title>
    <programlisting role="php">
<![CDATA[
<?php

abstract class A
{
    // Extending classes must have a publicly-gettable property
    abstract public string $readable {
        get;
    }

    // Extending classes must have a protected- or public-writeable property
    abstract protected string $writeable {
        set;
    }

    // Extending classes must have a protected or public symmetric property
    abstract protected string $both {
        get;
        set;
    }
}

class C extends A
{
    // This satisfies the requirement and also makes it settable, which is valid
    public string $readable;

    // This would NOT satisfy the requirement, as it is not publicly readable
    protected string $readable;

    // This satisfies the requirement exactly, so is sufficient.
    // It may only be written to, and only from protected scope
    protected string $writeable {
        set => $value;
    }

    // This expands the visibility from protected to public, which is fine
    public string $both;
}

?>
]]>
   </programlisting>
  </example>
  <simpara>
   An abstract property on an abstract class may provide implementations for any hook,
   but must have either <literal>get</literal> or <literal>set</literal> declared but not defined (as in the example above).
  </simpara>
  <example>
   <title>Abstract property with hooks example</title>
   <programlisting role="php" annotations="non-interactive">
<![CDATA[
<?php

abstract class A
{
    // This provides a default (but overridable) set implementation,
    // and requires child classes to provide a get implementation
    abstract public string $foo {
        get;

        set {
            $this->foo = $value;
        }
    }
}

?>
 ]]>
   </programlisting>
  </example>
 </sect1>
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