php-doc-en/language/constants.xml

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 <chapter id="language.constants">
  <title>Constants</title>

  <simpara>
   A constant is a identifier (name) for a simple value. As the name
   suggests, that value cannot change during the execution of the
   script.  (The 'magic constants' <constant>__FILE__</constant> and
   <constant>__LINE__</constant> appear to be an exception to this,
   but they're not actually constants.) A constant is case-sensitive
   by default. By convention constant identifiers are always
   uppercase.
  </simpara>
  <para>
   The name of a constant follows the same rules as any label in PHP. A 
   valid constant name starts with a letter or underscore, followed
   by any number of letters, numbers, or underscores. As a regular
   expression, it would be expressed thus:
   <literal>[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*</literal>

   <!-- TODO: Example of valid & invalid constant names -->
  </para>
				      
  <note>
   <simpara>
    For our purposes here, a letter is a-z, A-Z, and the ASCII
    characters from 127 through 255 (0x7f-0xff).
   </simpara>
  </note>

  <simpara>
   The scope of a constant is global--you can access it anywhere in
   your script without regard to scope.
  </simpara>

  <sect1 id="language.constants.syntax">
   <title>Syntax</title>
   <simpara>
    You can define a constant by using the 
    <function>define</function>-function. Once a constant is defined,
    it can never be changed or undefined.
   </simpara>
   <simpara>
    Only scalar data (<type>boolean</type>, <type>integer</type>, 
    <type>float</type> and <type>string</type>) can be contained 
    in constants.
   </simpara>
   <simpara>
    You can get the value of a constant by simply specifying its name.
    Unlike with variables, you should <emphasis>not</emphasis> prepend
    a constant with a <literal>$</literal>.
    You can also use the function <function>constant</function>, to
    read a constant's value, if you are to obtain the constant's name
    dynamically. 
    Use <function>get_defined_constants</function> to get a list of 
    all defined constants.
   </simpara>
   <note>
    <simpara>
     Constants and (global) variables are in a different namespace. 
     This implies that for example &true; and 
     <varname>$TRUE</varname> are generally different.
    </simpara>
   </note>
   <simpara>
    If you use an undefined constant, PHP assumes that you mean
    the name of the constant itself. A 
    <link linkend="features.error-handling">notice</link> will be issued
    when this happens. Use the <function>defined</function>-function if 
    you want to know if a constant is set.
   </simpara>
   <para>
    These are the differences between constants and variables:
    <itemizedlist>
     <listitem>
      <simpara>
       Constants do not have a dollar sign (<literal>$</literal>)
       before them;
      </simpara>
     </listitem>
     <listitem>
      <simpara>
       Constants may only be defined using the
       <function>define</function> function, not by simple assignment;
      </simpara>
     </listitem>
     <listitem>
      <simpara>
       Constants may be defined and accessed anywhere without regard
       to variable scoping rules;
      </simpara>
     </listitem>
     <listitem>
      <simpara>
       Constants may not be redefined or undefined once they have been
       set; and
      </simpara>
     </listitem>
     <listitem>
      <simpara>
       Constants may only evaluate to scalar values.
       </simpara>
     </listitem>
    </itemizedlist>
   </para>

   <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.
?>
]]>
     </programlisting>
    </example>

   </para>
  </sect1>
  
  <sect1 id="language.constants.predefined">
   <title>Predefined constants</title>

   <simpara>
    PHP provides a large number of predefined constants to any script
    which it runs. Many of these constants, however, are created by
    various extensions, and will only be present when those extensions
    are available, either via dynamic loading or because they have
    been compiled in.
   </simpara>

   <simpara>
    A list of predefined constants is available in the section <link
    linkend="reserved.constants">Predefined constants</link>.
   </simpara>


  </sect1>
 </chapter>
 
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