php-doc-en/language/constants.sgml

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

   <simpara>
    PHP defines several constants and provides a mechanism for
    defining more at run-time. Constants are much like variables,
    save for the two facts that constants must be defined using the
    <function>define</function> function, and that they cannot later be
    redefined to another value.

   <para>
	The predefined constants (always available) are:

	<variablelist>

	 <varlistentry>
	  <term>__FILE__</term>
	  <listitem>
	   <simpara>
		The name of the script file presently being parsed. If used
		within a file which has been included or required, then the
		name of the included file is given, and not the name of the
		parent file.
	   </simpara>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term>__LINE__</term>
	  <listitem>
	   <simpara>
		The number of the line within the current script file which is
		being parsed. If used within a file which has been included or
		required, then the position within the included file is given.
	   </simpara>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term>PHP_VERSION</term>
	  <listitem>
	   <simpara>
		The string representation of the version of the PHP parser
		presently in use; e.g. '3.0.8-dev'.
	   </simpara>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term>PHP_OS</term>
	  <listitem>
	   <simpara>
		The name of the operating system on which the PHP parser is
		executing; e.g. 'Linux'.
	   </simpara>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term>TRUE</term>
	  <listitem>
	   <simpara>
		A true value.
	   </simpara>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term>FALSE</term>
	  <listitem>
	   <simpara>
		A false value.
	   </simpara>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term>E_ERROR</term>
	  <listitem>
	   <simpara>
		Denotes an error other than a parsing error from which
		recovery is not possible. 
	   </simpara>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term>E_WARNING</term>
	  <listitem>
	   <simpara>
		Denotes a condition where PHP knows something is wrong, but
		will continue anyway; these can be caught by the script
		itself. An example would be an invalid regexp in
		<function>ereg</function>. 
	   </simpara>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term>E_PARSE</term>
	  <listitem>
	   <simpara>
		The parser choked on invalid syntax in the script
		file. Recovery is not possible.
	   </simpara>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term>E_NOTICE</term>
	  <listitem>
	   <simpara>
		Something happened which may or may not be an error. Execution 
		continues. Examples include using an unquoted string as a
		hash index, or accessing a variable which has not been set.
	   </simpara>
	  </listitem>
	 </varlistentry>
	</variablelist>

   <para>
	The E_* constants are typically used with the
	<function>error_reporting</function> function<6F>for setting the
	error reporting level.

   <para>
    You can define additional constants using the
    <function>define</function> function.
	
   <para>
	Note that these are constants, not C-style macros; only valid
	scalar data may be represented by a constant.

    <example>
     <title>Defining Constants</title>
     <programlisting>
&lt;?php
define("CONSTANT", "Hello world.");
echo CONSTANT; // outputs "Hello world."
?&gt;
     </programlisting>
    </example>

    <example>
     <title>Using __FILE__ and __LINE__</title>
     <programlisting>
&lt;?php
function report_error($file, $line, $message) {
    echo "An error occured in $file on line $line: $message.";
}

report_error(__FILE__,__LINE__, "Something went wrong!");
?&gt;
     </programlisting>
    </example>
	
  </chapter>
 
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