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Syntax for vim < 6 is set to sgml, because xml is not recognizing tag-names. This was discussed before, and this turned out to be best for all vi(m) versions. git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@57993 c90b9560-bf6c-de11-be94-00142212c4b1
290 lines
8.1 KiB
XML
290 lines
8.1 KiB
XML
<?xml encoding="iso-8859-1"?>
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<!-- $Revision: 1.18 $ -->
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<chapter id="language.constants">
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<title>Constants</title>
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<simpara>
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A constant is a identifier (name) for a simple value. As the name
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suggests, that value cannot change during the execution of the script
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(the magic constants <constant>__FILE__</constant> and
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<constant>__LINE__</constant> are the only exception). A constant
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is case-sensitive by default. By convention constants are always
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uppercase.
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</simpara>
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<para>
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The name of a constant follows the same rules as any label in PHP. A
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valid constant name starts with a letter or underscore, followed
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by any number of letters, numbers, or underscores. As a regular
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expression, it would be expressed thus:
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<literal>[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*</literal>
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<!-- TODO: Example of valid & invalid constant names -->
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</para>
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<note>
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<simpara>
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For our purposes here, a letter is a-z, A-Z, and the ASCII
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characters from 127 through 255 (0x7f-0xff).
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</simpara>
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</note>
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<simpara>
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The scope of a constant is global.
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</simpara>
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<sect1 id="language.constants.syntax">
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<title>Syntax</title>
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<simpara>
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You can define a constant by using the
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<function>define</function>-function. Once a constant is defined,
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it can never be changed or undefined.
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</simpara>
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<simpara>
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Only scalar data (<type>boolean</type>, <type>integer</type>,
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<type>double</type> and <type>string</type>) can be contained
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in constants.
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</simpara>
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<simpara>
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You can get the value of a constant by simply specifying its name.
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Unlike with variables, you should <emphasis>not</emphasis> prepend
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a constant with a <literal>$</literal>.
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You can also use the function <function>constant</function>, to
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read a constant's value, if you are to obtain the constant's name
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dynamically.
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Use <function>get_defined_constants</function> to get a list of
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all defined constants.
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</simpara>
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<note>
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<simpara>
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Constants and (global) variables are in a different namespace.
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This implies that for example &true; and
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<varname>$TRUE</varname> are generally different.
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</simpara>
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</note>
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<simpara>
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If you use an undefined constant, PHP assumes that you mean
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the name of the constant itself. A
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<link linkend="features.error-handling">notice</link> will be issued
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when this happens. Use the <function>defined</function>-function if
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you want to know if a constant is set.
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</simpara>
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<para>
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This are the differences with variables:
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<itemizedlist>
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<listitem>
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<simpara>
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Constants do not have a dollar sign (<literal>$</literal>) before them;
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</simpara>
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</listitem>
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<listitem>
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<simpara>
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Constants may be defined and accessed anywhere without regard to variable
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scoping rules;
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</simpara>
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</listitem>
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<listitem>
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<simpara>
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Constants may not be redefined or undefined once they have
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been set; and
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</simpara>
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</listitem>
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<listitem>
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<simpara>
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Constants may only evaluate to scalar values.
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</simpara>
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</listitem>
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</itemizedlist>
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</para>
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<para>
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<example>
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<title>Defining Constants</title>
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<programlisting>
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<?php
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define("CONSTANT", "Hello world.");
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echo CONSTANT; // outputs "Hello world."
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echo Constant; // outputs "Constant" and issues a notice.
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?>
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</programlisting>
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</example>
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</para>
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</sect1>
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<sect1 id="language.constants.predefined">
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<title>Predefined constants</title>
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<para>
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The predefined constants (always available) are:
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<variablelist>
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<varlistentry>
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<term>__FILE__ (case-insensitive)</term>
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<listitem>
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<simpara>
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The name of the script file presently being parsed. If used
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within a file which has been included or required, then the
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name of the included file is given, and not the name of the
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parent file.
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</simpara>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>__LINE__ (case-insensitive)</term>
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<listitem>
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<simpara>
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The number of the line within the current script file which is
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being parsed. If used within a file which has been included or
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required, then the position within the included file is given.
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</simpara>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>PHP_VERSION</term>
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<listitem>
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<simpara>
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The string representation of the version of the PHP parser
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presently in use; e.g. '4.0.7-dev'.
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</simpara>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>PHP_OS</term>
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<listitem>
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<simpara>
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The name of the operating system on which the PHP parser is
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executing;. Possible values may be :
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"AIX", "Darwin" (MacOS), "Linux", "SunOS", "WIN32", "WINNT".
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Note: other values may be available too.
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</simpara>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>&true; (case-insensitive)</term>
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<listitem>
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<simpara>
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A &true; value (see the <type>boolean</type> type).
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</simpara>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>&false; (case-insensitive)</term>
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<listitem>
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<simpara>
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A &false; value (see the <type>boolean</type> type).
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</simpara>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>&null; (case-insensitive)</term>
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<listitem>
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<simpara>
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A &null; value (see the <type>null</type> type).
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</simpara>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>E_ERROR</term>
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<listitem>
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<simpara>
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Denotes an error other than a parsing error from which
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recovery is not possible.
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</simpara>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>E_WARNING</term>
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<listitem>
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<simpara>
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Denotes a condition where PHP knows something is wrong, but
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will continue anyway; these can be caught by the script
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itself. An example would be an invalid regexp in
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<function>ereg</function>.
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</simpara>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>E_PARSE</term>
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<listitem>
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<simpara>
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The parser choked on invalid syntax in the script
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file. Recovery is not possible.
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</simpara>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>E_NOTICE</term>
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<listitem>
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<simpara>
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Something happened which may or may not be an error. Execution
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continues. Examples include using an unquoted string as an array
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index, or accessing a variable which has not been set.
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</simpara>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>E_ALL</term>
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<listitem>
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<simpara>
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All of the E_* constants rolled into one. If used with
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<function>error_reporting</function>, will cause any and all
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problems noticed by PHP to be reported.
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</simpara>
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</listitem>
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</varlistentry>
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</variablelist>
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</para>
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<para>
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The <literal>E_*</literal> constants are typically used with the
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<function>error_reporting</function> function for setting the
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error reporting level. See all these constants at
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<link linkend="features.error-handling">Error handling</link>.
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<example>
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<title>Using __FILE__ and __LINE__</title>
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<programlisting>
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<?php
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function report_error($file, $line, $message)
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{
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echo "An error occured in $file on line $line: $message.";
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}
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report_error(__FILE__, __LINE__, "Something went wrong!");
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?>
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</programlisting>
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</example>
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</para>
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</sect1>
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</chapter>
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<!-- Keep this comment at the end of the file
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Local variables:
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mode: sgml
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sgml-omittag:t
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sgml-shorttag:t
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sgml-minimize-attributes:nil
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sgml-always-quote-attributes:t
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sgml-indent-step:1
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sgml-indent-data:t
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sgml-parent-document:nil
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sgml-default-dtd-file:"../../manual.ced"
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sgml-exposed-tags:nil
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sgml-local-catalogs:nil
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sgml-local-ecat-files:nil
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End:
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vim600: syn=xml fen fdm=syntax fdl=2 si
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vim: et tw=78 syn=sgml
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vi: ts=1 sw=1
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-->
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