<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >Class Constants</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK REL="HOME" TITLE="PHP 手册" HREF="index.html"><LINK REL="UP" TITLE="类与对象(PHP 5)" HREF="language.oop5.html"><LINK REL="PREVIOUS" TITLE="Static Keyword" HREF="language.oop5.static.html"><LINK REL="NEXT" TITLE="Class Abstraction" HREF="language.oop5.abstract.html"><META HTTP-EQUIV="Content-type" CONTENT="text/html; charset=UTF-8"></HEAD ><BODY CLASS="sect1" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#840084" ALINK="#0000FF" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" >PHP 手册</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="language.oop5.static.html" ACCESSKEY="P" >上一页</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" >章 19. 类与对象(PHP 5)</TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="language.oop5.abstract.html" ACCESSKEY="N" >下一页</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="sect1" ><H1 CLASS="sect1" ><A NAME="language.oop5.constants" >Class Constants</A ></H1 ><P > It is possible to define constant values on a per-class basis remaining the same and unchangeable. Constants differ from normal variables in that you don't use the <CODE CLASS="varname" >$</CODE > symbol to declare or use them. Like <A HREF="language.oop5.static.html" >static</A > members, constant values cannot be accessed from an instance of the object (using <TT CLASS="literal" >$object::constant</TT >). </P ><P > The value must be a constant expression, not (for example) a variable, a class member, result of a mathematical operation or a function call. </P ><TABLE WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" CLASS="EXAMPLE" ><TR ><TD ><DIV CLASS="example" ><A NAME="AEN5853" ></A ><P ><B >例 19-17. Defining and using a constant</B ></P ><TABLE BORDER="0" BGCOLOR="#E0E0E0" CELLPADDING="5" ><TR ><TD ><code><font color="#000000"> <font color="#0000BB"><?php<br /></font><font color="#007700">class </font><font color="#0000BB">MyClass<br /></font><font color="#007700">{<br /> const </font><font color="#0000BB">constant </font><font color="#007700">= </font><font color="#DD0000">'constant value'</font><font color="#007700">;<br /><br /> function </font><font color="#0000BB">showConstant</font><font color="#007700">() {<br /> echo </font><font color="#0000BB">self</font><font color="#007700">::</font><font color="#0000BB">constant </font><font color="#007700">. </font><font color="#DD0000">"\n"</font><font color="#007700">;<br /> }<br />}<br /><br />echo </font><font color="#0000BB">MyClass</font><font color="#007700">::</font><font color="#0000BB">constant </font><font color="#007700">. </font><font color="#DD0000">"\n"</font><font color="#007700">;<br /><br /></font><font color="#0000BB">$class </font><font color="#007700">= new </font><font color="#0000BB">MyClass</font><font color="#007700">();<br /></font><font color="#0000BB">$class</font><font color="#007700">-></font><font color="#0000BB">showConstant</font><font color="#007700">();<br /></font><font color="#FF8000">// echo $class::constant; is not allowed<br /></font><font color="#0000BB">?></font> </font> </code></TD ></TR ></TABLE ></DIV ></TD ></TR ></TABLE ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="language.oop5.static.html" ACCESSKEY="P" >上一页</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >起始页</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="language.oop5.abstract.html" ACCESSKEY="N" >下一页</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Static Keyword</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="language.oop5.html" ACCESSKEY="U" >上一级</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Class Abstraction</TD ></TR ></TABLE ></DIV ></BODY ></HTML >