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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>NULL</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.types.resource.html">Resources</a></div> <div class="next" style="text-align: right; float: right;"><a href="language.pseudo-types.html">Pseudo-types and variables used in this documentation</a></div> <div class="up"><a href="language.types.html">Types</a></div> <div class="home"><a href="index.html">PHP Manual</a></div></div><hr /><div id="language.types.null" class="sect1"> <h2 class="title">NULL</h2> <p class="para"> The special <b><tt>NULL</tt></b> value represents a variable with no value. <b><tt>NULL</tt></b> is the only possible value of type <a href="language.types.null.html" class="type NULL">NULL</a>. </p> <blockquote><p><b class="note">Note</b>: <span class="simpara"> The <a href="language.types.null.html" class="type null">null</a> type was introduced in PHP 4. </span> </p></blockquote> <p class="para"> A variable is considered to be <a href="language.types.null.html" class="type null">null</a> if: </p> <ul class="itemizedlist"> <li class="listitem"> <p class="para"> it has been assigned the constant <b><tt>NULL</tt></b>. </p> </li> <li class="listitem"> <p class="para"> it has not been set to any value yet. </p> </li> <li class="listitem"> <p class="para"> it has been <a href="function.unset.html" class="function">unset()</a>. </p> </li> </ul> <div id="language.types.null.syntax" class="sect2"> <h3 class="title">Syntax</h3> <p class="para"> There is only one value of type <a href="language.types.null.html" class="type null">null</a>, and that is the case-insensitive keyword <b><tt>NULL</tt></b>. </p> <div class="informalexample"> <div class="example-contents"><div class="phpcode"><code><span style="color: #000000"><span style="color: #0000BB"><?php<br />$var </span><span style="color: #007700">= </span><span style="color: #0000BB">NULL</span><span style="color: #007700">; <br /></span><span style="color: #0000BB">?></span></span></code></div> </div> </div> <p class="para"> See also the functions <a href="function.is-null.html" class="function">is_null()</a> and <a href="function.unset.html" class="function">unset()</a>. </p> </div> <div id="language.types.null.casting" class="sect2"> <h3 class="title">Casting to <i>NULL</i></h3> <p class="para"> Casting a variable to <a href="language.types.null.html" class="type null">null</a> will remove the variable and unset its value. </p> </div></div><hr /><div style="text-align: center;"> <div class="prev" style="text-align: left; float: left;"><a href="language.types.resource.html">Resources</a></div> <div class="next" style="text-align: right; float: right;"><a href="language.pseudo-types.html">Pseudo-types and variables used in this documentation</a></div> <div class="up"><a href="language.types.html">Types</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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