language/oop5/inheritance.xml
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many classes and objects relate to one another.
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</para>
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<para>
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For example, when you extend a class, the subclass inherits all of the
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public and protected methods from the parent class. Unless a class overrides
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For example, when extending a class, the subclass inherits all of the
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public and protected methods, properties and constants from the parent class.
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Unless a class overrides
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those methods, they will retain their original functionality.
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</para>
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<para>
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implementation of additional functionality in similar objects without the
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need to reimplement all of the shared functionality.
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</para>
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<para>
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Private methods of a parent class are not accessible to a child class. As a result,
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child classes may reimplement a private method themselves without regard for normal
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inheritance rules. Prior to PHP 8.0.0, however, <literal>final</literal> and <literal>static</literal>
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restrictions were applied to private methods. As of PHP 8.0.0, the only private method
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restriction that is enforced is <literal>private final</literal> constructors, as that
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is a common way to "disable" the constructor when using static factory methods instead.
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</para>
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<para>
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The <link linkend="language.oop5.visibility">visibility</link>
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of methods, properties and constants can be relaxed, e.g. a
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<literal>protected</literal> method can be marked as
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<literal>public</literal>, but they cannot be restricted, e.g.
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marking a <literal>public</literal> property as <literal>private</literal>.
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An exception are constructors, whose visibility can be restricted, e.g.
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a <literal>public</literal> constructor can be marked as <literal>private</literal>
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in a child class.
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</para>
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<note>
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<para>
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Unless autoloading is used, then classes must be defined before they are
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Unless autoloading is used, the classes must be defined before they are
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used. If a class extends another, then the parent class must be declared
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before the child class structure. This rule applies to classes that inherit
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other classes and interfaces.
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</para>
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</note>
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<sect2 xml:id="language.oop5.inheritance.examples">
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<example xml:id="language.oop5.inheritance.examples.ex1">
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<title>Inheritance Example</title>
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<note>
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<para>
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It is not allowed to override a read-write property with a <link linkend="language.oop5.properties.readonly-properties">readonly property</link> or vice versa.
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<informalexample>
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<programlisting role="php">
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<![CDATA[
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<?php
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class A {
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public int $prop;
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}
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class B extends A {
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// Illegal: read-write -> readonly
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public readonly int $prop;
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}
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?>
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]]>
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</programlisting>
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</informalexample>
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</para>
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</note>
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<example>
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<title>Inheritance Example</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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class Foo
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{
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public function printItem($string)
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?>
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]]>
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</programlisting>
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</example>
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<sect2 xml:id="language.oop5.inheritance.internal-classes">
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<title>Return Type Compatibility with Internal Classes</title>
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<para>
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Prior to PHP 8.1, most internal classes or methods didn't declare their return types,
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and any return type was allowed when extending them.
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</para>
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<para>
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As of PHP 8.1.0, most internal methods started to "tentatively" declare their return type,
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in that case the return type of methods should be compatible with the parent being extended;
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otherwise, a deprecation notice is emitted.
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Note that lack of an explicit return declaration is also considered a signature mismatch,
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and thus results in the deprecation notice.
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</para>
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<para>
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If the return type cannot be declared for an overriding method due to PHP cross-version compatibility concerns,
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a <classname>ReturnTypeWillChange</classname> attribute can be added to silence the deprecation notice.
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</para>
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<example>
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<title>The overriding method does not declare any return type</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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class MyDateTime extends DateTime
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{
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public function modify(string $modifier) { return false; }
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}
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// "Deprecated: Return type of MyDateTime::modify(string $modifier) should either be compatible with DateTime::modify(string $modifier): DateTime|false, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice" as of PHP 8.1.0
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?>
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]]>
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</programlisting>
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</example>
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<example>
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<title>The overriding method declares a wrong return type</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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class MyDateTime extends DateTime
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{
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public function modify(string $modifier): ?DateTime { return null; }
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}
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// "Deprecated: Return type of MyDateTime::modify(string $modifier): ?DateTime should either be compatible with DateTime::modify(string $modifier): DateTime|false, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice" as of PHP 8.1.0
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?>
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]]>
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</programlisting>
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</example>
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<example>
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<title>The overriding method declares a wrong return type without a deprecation notice</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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class MyDateTime extends DateTime
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{
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/**
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* @return DateTime|false
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*/
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#[\ReturnTypeWillChange]
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public function modify(string $modifier) { return false; }
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}
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// No notice is triggered
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?>
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]]>
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</programlisting>
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</example>
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</sect2>
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</sect1>
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