php-doc-en/language/oop5/final.xml

<?xml version="1.0" encoding="utf-8"?>
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 <sect1 xml:id="language.oop5.final" xmlns="http://docbook.org/ns/docbook">
  <title>Final Keyword</title>
  <para>
   The final keyword prevents child classes from overriding a method or constant by
   prefixing the definition with <literal>final</literal>. If the class
   itself is being defined final then it cannot be extended.
  </para>
  <para>
   <example>
    <title>Final methods example</title>
    <programlisting role="php">
<![CDATA[
<?php
class BaseClass {
   public function test() {
       echo "BaseClass::test() called\n";
   }
   
   final public function moreTesting() {
       echo "BaseClass::moreTesting() called\n";
   }
}

class ChildClass extends BaseClass {
   public function moreTesting() {
       echo "ChildClass::moreTesting() called\n";
   }
}
// Results in Fatal error: Cannot override final method BaseClass::moreTesting()
?> 
]]>
   </programlisting>
  </example>
  </para>
  <para>
   <example>
    <title>Final class example</title>
    <programlisting role="php">
<![CDATA[
<?php
final class BaseClass {
   public function test() {
       echo "BaseClass::test() called\n";
   }

   // As the class is already final, the final keyword is redundant
   final public function moreTesting() {
       echo "BaseClass::moreTesting() called\n";
   }
}

class ChildClass extends BaseClass {
}
// Results in Fatal error: Class ChildClass may not inherit from final class (BaseClass)
?> 
]]>
    </programlisting>
   </example>
  </para>
  <para>
   <example xml:id="language.oop5.final.example.php81">
    <title>Final constants example as of PHP 8.1.0</title>
    <programlisting role="php">
<![CDATA[
<?php
class Foo
{
    final public const X = "foo";
}

class Bar extends Foo
{
    public const X = "bar";
}

// Fatal error: Bar::X cannot override final constant Foo::X
?>
]]>
    </programlisting>
   </example>
  </para>

  <note>
   <simpara>
    Properties cannot be declared final: only classes, methods, and constants (as of PHP 8.1.0) may be declared as final.
   </simpara>
   <simpara>
    As of PHP 8.0.0, private methods may not be declared final except for the constructor.
   </simpara>
  </note>
 </sect1>
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