define.xml

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<refentry xml:id="function.define" xmlns="http://docbook.org/ns/docbook">
 <refnamediv>
  <refname>define</refname>
  <refpurpose>Defines a named constant</refpurpose>
 </refnamediv>
 
 <refsect1 role="description">
  &reftitle.description;
  <methodsynopsis>
   <type>bool</type><methodname>define</methodname>
   <methodparam><type>string</type><parameter>constant_name</parameter></methodparam>
   <methodparam><type>mixed</type><parameter>value</parameter></methodparam>
   <methodparam choice="opt"><type>bool</type><parameter>case_insensitive</parameter><initializer>&false;</initializer></methodparam>
  </methodsynopsis>
  <para>
   Defines a named constant at runtime.
  </para>
 </refsect1>

 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>constant_name</parameter></term>
     <listitem>
      <para>
       The name of the constant.
      </para>
      <note>
       <para>
        It is possible to <function>define</function> constants with reserved or
        even invalid names, whose value can (only) be retrieved with
        <function>constant</function>. However, doing so is not recommended.
       </para>
      </note>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>value</parameter></term>
     <listitem>
      <para>
       The value of the constant. In PHP 5, <parameter>value</parameter> must
       be a <type>scalar</type> value (<type>int</type>,
       <type>float</type>, <type>string</type>, <type>bool</type>, or
       &null;). In PHP 7, <type>array</type> values are also accepted.
      </para>
      <warning>
       <para>
        While it is possible to define <type>resource</type> constants, it is
        not recommended and may cause unpredictable behavior.
       </para>
      </warning>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>case_insensitive</parameter></term>
     <listitem>
      <para>
       If set to &true;, the constant will be defined case-insensitive. 
       The default behavior is case-sensitive; i.e. 
       <literal>CONSTANT</literal> and <literal>Constant</literal> represent
       different values.
      </para>
      <warning>
       <simpara>
        Defining case-insensitive constants is deprecated as of PHP 7.3.0.
        As of PHP 8.0.0, only &false; is an acceptable value, passing
        &true; will produce a warning.
       </simpara>
      </warning>
      <note>
       <para>
        Case-insensitive constants are stored as lower-case.
       </para>
      </note>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
 </refsect1>

 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   &return.success;
  </para>
 </refsect1>

 <refsect1 role="changelog">
  &reftitle.changelog;
  <para>
   <informaltable>
    <tgroup cols="2">
     <thead>
      <row>
       <entry>&Version;</entry>
       <entry>&Description;</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry>8.0.0</entry>
       <entry>
        Passing &true; to <parameter>case_insensitive</parameter> now emits an <constant>E_WARNING</constant>. Passing &false; is still allowed.
       </entry>
      </row>
      <row>
       <entry>7.3.0</entry>
       <entry>
        <parameter>case_insensitive</parameter> has been deprecated and will be removed in version 8.0.0.
       </entry>
      </row>
      <row>
       <entry>7.0.0</entry>
       <entry>
        <type>array</type> values are allowed.
       </entry>
      </row>
     </tbody>
    </tgroup>
   </informaltable>
  </para>
 </refsect1>
 
 <refsect1 role="examples">
  &reftitle.examples;
  <para>
   <example>
    <title>Defining Constants</title>
    <programlisting role="php">
<![CDATA[
<?php
define("CONSTANT", "Hello world.");
echo CONSTANT; // outputs "Hello world."
echo Constant; // outputs "Constant" and issues a notice.

define("GREETING", "Hello you.", true);
echo GREETING; // outputs "Hello you."
echo Greeting; // outputs "Hello you."

// Works as of PHP 7
define('ANIMALS', array(
    'dog',
    'cat',
    'bird'
));
echo ANIMALS[1]; // outputs "cat"

?>
]]>
    </programlisting>
   </example>
  </para>
  <para>
   <example>
    <title>Constants with Reserved Names</title>
    <para>
     This example illustrates the <emphasis>possibility</emphasis> to define a
     constant with the same name as a
     <link linkend="language.constants.predefined">magic constant</link>.
     Since the resulting behavior is obviously confusing, it is not recommended
     to do this in practise, though.
    </para>
    <programlisting role="php">
<![CDATA[
<?php
var_dump(defined('__LINE__'));
var_dump(define('__LINE__', 'test'));
var_dump(constant('__LINE__'));
var_dump(__LINE__);
?>
]]>
    </programlisting>
   &example.outputs;
   <screen>
<![CDATA[
bool(false)
bool(true)
string(4) "test"
int(5)
]]>
   </screen>
   </example>
  </para>
 </refsect1>

 <refsect1 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member><function>defined</function></member>
    <member><function>constant</function></member>
    <member>The section on <link linkend="language.constants">Constants</link></member>
   </simplelist>
  </para>
 </refsect1>

</refentry>
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