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<?xml version="1.0" encoding="utf-8"?>
<sect1 xml:id="language.types.type-system">
<title>Type System</title>
<para>
PHP uses a nominal type system with a strong behavioral subtyping relation.
The subtyping relation is checked at compile time whereas the verification of
types is dynamically checked at run time.
</para>
<para>
PHP's type system supports various atomic types that can be composed together
to create more complex types. Some of these types can be written as
<link linkend="language.types.declarations">type declarations</link>.
</para>
<sect2 xml:id="language.types.type-system.atomic">
<title>Atomic types</title>
<para>
Some atomic types are built-in types which are tightly integrated with the
language and cannot be reproduced with user defined types.
</para>
<para>
The list of base types is:
<itemizedlist>
<listitem>
<simpara>Built-in types</simpara>
<itemizedlist>
<listitem>
<simpara><type>null</type> type</simpara>
</listitem>
<listitem>
<simpara>
Scalar types:
</simpara>
<itemizedlist>
<listitem>
<simpara><type>bool</type> type</simpara>
</listitem>
<listitem>
<simpara><type>int</type> type</simpara>
</listitem>
<listitem>
<simpara><type>float</type> type</simpara>
</listitem>
<listitem>
<simpara><type>string</type> type</simpara>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<simpara><type>array</type> type</simpara>
</listitem>
<listitem>
<simpara><type>object</type> type</simpara>
</listitem>
<listitem>
<simpara><type>resource</type> type</simpara>
</listitem>
<listitem>
<simpara><type>never</type> type</simpara>
</listitem>
<listitem>
<simpara><type>void</type> type</simpara>
</listitem>
<listitem>
<simpara>
<link linkend="language.types.relative-class-types">Relative class types</link>:
<type>self</type>, <type>parent</type>, and <type>static</type>
</simpara>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<simpara>
<link linkend="language.types.value">Value types</link>
</simpara>
<itemizedlist>
<listitem>
<simpara><type>false</type></simpara>
</listitem>
<listitem>
<simpara><type>true</type></simpara>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<simpara>
User-defined types (generally referred to as class-types)
</simpara>
<itemizedlist>
<listitem>
<simpara><link linkend="language.oop5.interfaces">Interfaces</link></simpara>
</listitem>
<listitem>
<simpara><link linkend="language.oop5.basic.class">Classes</link></simpara>
</listitem>
<listitem>
<simpara><link linkend="language.types.enumerations">Enumerations</link></simpara>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<simpara><type>callable</type> type</simpara>
</listitem>
</itemizedlist>
</para>
<sect3 xml:id="language.types.type-system.atomic.scalar">
<title>Scalar types</title>
<simpara>
A value is considered scalar if it is of type <type>int</type>,
<type>float</type>, <type>string</type> or <type>bool</type>.
</simpara>
</sect3>
<sect3 xml:id="language.types.type-system.atomic.user-defined">
<title>User-defined types</title>
<simpara>
It is possible to define custom types with
<link linkend="language.oop5.interfaces">interfaces</link>,
<link linkend="language.oop5.basic.class">classes</link> and
<link linkend="language.types.enumerations">enumerations</link>.
These are considered as user-defined types, or class-types.
For example, a class called <literal>Elephant</literal> can be defined,
then objects of type <literal>Elephant</literal> can be instantiated,
and a function can request a parameter of type <literal>Elephant</literal>.
</simpara>
</sect3>
</sect2>
<sect2 xml:id="language.types.type-system.composite">
<title>Composite types</title>
<para>
It is possible to combine multiple atomic types into composite types.
PHP allows types to be combined in the following ways:
</para>
<itemizedlist>
<listitem>
<simpara>
Intersection of class-types (interfaces and class names).
</simpara>
</listitem>
<listitem>
<simpara>
Union of types.
</simpara>
</listitem>
</itemizedlist>
<sect3 xml:id="language.types.type-system.composite.intersection">
<title>Intersection types</title>
<para>
An intersection type accepts values which satisfies multiple
class-type declarations, rather than a single one.
Individual types which form the intersection type are joined by the
<literal>&</literal> symbol. Therefore, an intersection type comprised
of the types <literal>T</literal>, <literal>U</literal>, and
<literal>V</literal> will be written as <literal>T&U&V</literal>.
</para>
</sect3>
<sect3 xml:id="language.types.type-system.composite.union">
<title>Union types</title>
<para>
A union type accepts values of multiple different types,
rather than a single one.
Individual types which form the union type are joined by the
<literal>|</literal> symbol. Therefore, a union type comprised
of the types <literal>T</literal>, <literal>U</literal>, and
<literal>V</literal> will be written as <literal>T|U|V</literal>.
If one of the types is an intersection type, it needs to be bracketed
with parenthesis for it to written in <acronym>DNF</acronym>:
<literal>T|(X&Y)</literal>.
</para>
</sect3>
</sect2>
<sect2 xml:id="language.types.type-system.alias">
<title>Type aliases</title>
<para>
PHP supports two type aliases: <type>mixed</type> and
<type>iterable</type> which corresponds to the
<link linkend="language.types.type-system.composite.union">union type</link>
of <literal>object|resource|array|string|float|int|bool|null</literal>
and <literal>Traversable|array</literal> respectively.
</para>
<note>
<simpara>
PHP does not support user-defined type aliases.
</simpara>
</note>
</sect2>
</sect1>
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