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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"><html> <head> <title>Constants</title> <meta http-equiv="content-type" content="text/html; charset=UTF-8"> </head> <body><div style="text-align: center;"> <div class="prev" style="text-align: left; float: left;"><a href="language.variables.external.html">Variables From External Sources</a></div> <div class="next" style="text-align: right; float: right;"><a href="language.constants.predefined.html">Magic constants</a></div> <div class="up"><a href="langref.html">Language Reference</a></div> <div class="home"><a href="index.html">PHP Manual</a></div></div><hr /><div> <h1>Constants</h1><h2>Table of Contents</h2><ul class="chunklist chunklist_chapter"><li><a href="language.constants.predefined.html">Magic constants</a></li></ul> <p class="simpara"> A constant is an identifier (name) for a simple value. As the name suggests, that value cannot change during the execution of the script (except for <a href="language.constants.predefined.html" class="link"> magic constants</a>, which aren't actually constants). A constant is case-sensitive by default. By convention, constant identifiers are always uppercase. </p> <p class="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 thusly: <i>[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*</i> </p> <div class="tip"><b class="tip">Tip</b><p class="simpara">See also the<a href="userlandnaming.html" class="xref">Userland Naming Guide</a>.</p></div> <p class="para"> <div class="example"> <p><b>Example #1 Valid and invalid constant names</b></p> <div class="example-contents"><div class="phpcode"><code><span style="color: #000000"><span style="color: #0000BB"><?php<br /><br /></span><span style="color: #FF8000">// Valid constant names<br /></span><span style="color: #0000BB">define</span><span style="color: #007700">(</span><span style="color: #DD0000">"FOO"</span><span style="color: #007700">, </span><span style="color: #DD0000">"something"</span><span style="color: #007700">);<br /></span><span style="color: #0000BB">define</span><span style="color: #007700">(</span><span style="color: #DD0000">"FOO2"</span><span style="color: #007700">, </span><span style="color: #DD0000">"something else"</span><span style="color: #007700">);<br /></span><span style="color: #0000BB">define</span><span style="color: #007700">(</span><span style="color: #DD0000">"FOO_BAR"</span><span style="color: #007700">, </span><span style="color: #DD0000">"something more"</span><span style="color: #007700">);<br /><br /></span><span style="color: #FF8000">// Invalid constant names<br /></span><span style="color: #0000BB">define</span><span style="color: #007700">(</span><span style="color: #DD0000">"2FOO"</span><span style="color: #007700">, </span><span style="color: #DD0000">"something"</span><span style="color: #007700">);<br /><br /></span><span style="color: #FF8000">// This is valid, but should be avoided:<br />// PHP may one day provide a magical constant<br />// that will break your script<br /></span><span style="color: #0000BB">define</span><span style="color: #007700">(</span><span style="color: #DD0000">"__FOO__"</span><span style="color: #007700">, </span><span style="color: #DD0000">"something"</span><span style="color: #007700">); <br /><br /></span><span style="color: #0000BB">?></span></span></code></div> </div> </div> </p> <blockquote><p><b class="note">Note</b>: <span class="simpara"> For our purposes here, a letter is a-z, A-Z, and the ASCII characters from 127 through 255 (0x7f-0xff). </span> </p></blockquote> <p class="simpara"> Like <a href="language.variables.predefined.html" class="link">superglobals</a>, the scope of a constant is global. You can access constants anywhere in your script without regard to scope. For more information on scope, read the manual section on <a href="language.variables.scope.html" class="link">variable scope</a>. </p> <div id="language.constants.syntax" class="sect1"> <h2 class="title">Syntax</h2> <p class="simpara"> You can define a constant by using the <a href="function.define.html" class="function">define()</a>-function. Once a constant is defined, it can never be changed or undefined. </p> <p class="simpara"> Only scalar data (<a href="language.types.boolean.html" class="type boolean">boolean</a>, <a href="language.types.integer.html" class="type integer">integer</a>, <a href="language.types.float.html" class="type float">float</a> and <a href="language.types.string.html" class="type string">string</a>) can be contained in constants. Do not define <a href="language.types.resource.html" class="type resource">resource</a> constants. </p> <p class="simpara"> You can get the value of a constant by simply specifying its name. Unlike with variables, you should <em class="emphasis">not</em> prepend a constant with a <i>$</i>. You can also use the function <a href="function.constant.html" class="function">constant()</a> to read a constant's value if you wish to obtain the constant's name dynamically. Use <a href="function.get-defined-constants.html" class="function">get_defined_constants()</a> to get a list of all defined constants. </p> <blockquote><p><b class="note">Note</b>: <span class="simpara"> Constants and (global) variables are in a different namespace. This implies that for example <b><tt>TRUE</tt></b> and <var class="varname">$TRUE</var> are generally different. </span> </p></blockquote> <p class="simpara"> If you use an undefined constant, PHP assumes that you mean the name of the constant itself, just as if you called it as a <a href="language.types.string.html" class="type string">string</a> (CONSTANT vs "CONSTANT"). An error of level <a href="ref.errorfunc.html" class="link">E_NOTICE</a> will be issued when this happens. See also the manual entry on why <a href="language.types.array.html#language.types.array.foo-bar" class="link">$foo[bar]</a> is wrong (unless you first <a href="function.define.html" class="function">define()</a> <i>bar</i> as a constant). If you simply want to check if a constant is set, use the <a href="function.defined.html" class="function">defined()</a> function. </p> <p class="para"> These are the differences between constants and variables: <ul class="itemizedlist"> <li class="listitem"> <span class="simpara"> Constants do not have a dollar sign (<i>$</i>) before them; </span> </li> <li class="listitem"> <span class="simpara"> Constants may only be defined using the <a href="function.define.html" class="function">define()</a> function, not by simple assignment; </span> </li> <li class="listitem"> <span class="simpara"> Constants may be defined and accessed anywhere without regard to variable scoping rules; </span> </li> <li class="listitem"> <span class="simpara"> Constants may not be redefined or undefined once they have been set; and </span> </li> <li class="listitem"> <span class="simpara"> Constants may only evaluate to scalar values. </span> </li> </ul> </p> <p class="para"> <div class="example"> <p><b>Example #2 Defining Constants</b></p> <div class="example-contents"><div class="phpcode"><code><span style="color: #000000"><span style="color: #0000BB"><?php<br />define</span><span style="color: #007700">(</span><span style="color: #DD0000">"CONSTANT"</span><span style="color: #007700">, </span><span style="color: #DD0000">"Hello world."</span><span style="color: #007700">);<br />echo </span><span style="color: #0000BB">CONSTANT</span><span style="color: #007700">; </span><span style="color: #FF8000">// outputs "Hello world."<br /></span><span style="color: #007700">echo </span><span style="color: #0000BB">Constant</span><span style="color: #007700">; </span><span style="color: #FF8000">// outputs "Constant" and issues a notice.<br /></span><span style="color: #0000BB">?></span></span></code></div> </div> </div> </p> <p class="simpara"> See also <a href="language.oop5.constants.html" class="link">Class Constants</a>. </p> </div> </div><hr /><div style="text-align: center;"> <div class="prev" style="text-align: left; float: left;"><a href="language.variables.external.html">Variables From External Sources</a></div> <div class="next" style="text-align: right; float: right;"><a href="language.constants.predefined.html">Magic constants</a></div> <div class="up"><a href="langref.html">Language Reference</a></div> <div class="home"><a href="index.html">PHP Manual</a></div></div></body></html>⌨️ 快捷键说明
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