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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>Variable variables</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.scope.html">Variable scope</a></div> <div class="next" style="text-align: right; float: right;"><a href="language.variables.external.html">Variables From External Sources</a></div> <div class="up"><a href="language.variables.html">Variables</a></div> <div class="home"><a href="index.html">PHP Manual</a></div></div><hr /><div id="language.variables.variable" class="sect1"> <h2 class="title">Variable variables</h2> <p class="simpara"> Sometimes it is convenient to be able to have variable variable names. That is, a variable name which can be set and used dynamically. A normal variable is set with a statement such as: </p> <div class="informalexample"> <div class="example-contents"><div class="phpcode"><code><span style="color: #000000"><span style="color: #0000BB"><?php<br />$a </span><span style="color: #007700">= </span><span style="color: #DD0000">'hello'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?></span></span></code></div> </div> </div> <p class="simpara"> A variable variable takes the value of a variable and treats that as the name of a variable. In the above example, <em class="emphasis">hello</em>, can be used as the name of a variable by using two dollar signs. i.e. </p> <div class="informalexample"> <div class="example-contents"><div class="phpcode"><code><span style="color: #000000"><span style="color: #0000BB"><?php<br /></span><span style="color: #007700">$</span><span style="color: #0000BB">$a </span><span style="color: #007700">= </span><span style="color: #DD0000">'world'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?></span></span></code></div> </div> </div> <p class="simpara"> At this point two variables have been defined and stored in the PHP symbol tree: <var class="varname">$a</var> with contents "hello" and <var class="varname">$hello</var> with contents "world". Therefore, this statement: </p> <div class="informalexample"> <div class="example-contents"><div class="phpcode"><code><span style="color: #000000"><span style="color: #0000BB"><?php<br /></span><span style="color: #007700">echo </span><span style="color: #DD0000">"$a ${$a}"</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?></span></span></code></div> </div> </div> <p class="simpara"> produces the exact same output as: </p> <div class="informalexample"> <div class="example-contents"><div class="phpcode"><code><span style="color: #000000"><span style="color: #0000BB"><?php<br /></span><span style="color: #007700">echo </span><span style="color: #DD0000">"$a $hello"</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?></span></span></code></div> </div> </div> <p class="simpara"> i.e. they both produce: <span class="computeroutput">hello world</span>. </p> <p class="simpara"> In order to use variable variables with arrays, you have to resolve an ambiguity problem. That is, if you write <var class="varname">$$a[1]</var> then the parser needs to know if you meant to use <var class="varname">$a[1]</var> as a variable, or if you wanted <var class="varname">$$a</var> as the variable and then the [1] index from that variable. The syntax for resolving this ambiguity is: <var class="varname">${$a[1]}</var> for the first case and <var class="varname">${$a}[1]</var> for the second. </p> <div class="warning"><b class="warning">Warning</b> <p class="simpara"> Please note that variable variables cannot be used with PHP's <a href="language.variables.superglobals.html" class="link">Superglobal arrays</a> within functions or class methods. The variable <i>$this</i> is also a special variable that cannot be referenced dynamically. </p> </div> </div><hr /><div style="text-align: center;"> <div class="prev" style="text-align: left; float: left;"><a href="language.variables.scope.html">Variable scope</a></div> <div class="next" style="text-align: right; float: right;"><a href="language.variables.external.html">Variables From External Sources</a></div> <div class="up"><a href="language.variables.html">Variables</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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