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>class_exists</H1
><DIV
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><A
NAME="AEN16800"
></A
><P
>    (PHP 4, PHP 5)</P
>class_exists&nbsp;--&nbsp;检查类是否已定义</DIV
><DIV
CLASS="refsect1"
><A
NAME="AEN16803"
></A
><H2
>说明</H2
>bool <B
CLASS="methodname"
>class_exists</B
> ( string class_name [, bool autoload] )<BR
></BR
><P
>&#13;   如果由 <CODE
CLASS="parameter"
>class_name</CODE
>
   所指的类已经定义，此函数返回 
   <TT
CLASS="constant"
><B
>TRUE</B
></TT
>，否则返回 <TT
CLASS="constant"
><B
>FALSE</B
></TT
>。
  </P
><P
>&#13;   <TABLE
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><DIV
CLASS="example"
><A
NAME="AEN16819"
></A
><P
><B
>例 1. <B
CLASS="function"
>class_exists()</B
> 例子</B
></P
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><code><font color="#000000">
<font color="#0000BB">&lt;?php<br /></font><font color="#FF8000">// Check the class exists before trying to use it<br /></font><font color="#007700">if (</font><font color="#0000BB">class_exists</font><font color="#007700">(</font><font color="#DD0000">'MyClass'</font><font color="#007700">)) {<br />&nbsp;&nbsp;&nbsp;&nbsp;</font><font color="#0000BB">$myclass </font><font color="#007700">= new </font><font color="#0000BB">MyClass</font><font color="#007700">();<br />}<br /><br /></font><font color="#0000BB">?&gt;</font>
</font>
</code></TD
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>
  </P
><P
>&#13;   <B
CLASS="function"
>class_exists()</B
> 默认将会尝试调用 <A
HREF="language.oop5.autoload.html"
>__autoload</A
>，如果不想让
   <B
CLASS="function"
>class_exists()</B
> 调用
   <A
HREF="language.oop5.autoload.html"
>__autoload</A
>，可以将 <CODE
CLASS="parameter"
>autoload</CODE
> 参数设为 <TT
CLASS="constant"
><B
>FALSE</B
></TT
>。
  </P
><P
>&#13;   <TABLE
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><TR
><TD
><DIV
CLASS="example"
><A
NAME="AEN16831"
></A
><P
><B
>例 2. <CODE
CLASS="parameter"
>autoload</CODE
> 参数例子</B
></P
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><code><font color="#000000">
<font color="#0000BB">&lt;?php<br /></font><font color="#007700">function </font><font color="#0000BB">__autoload</font><font color="#007700">(</font><font color="#0000BB">$class</font><font color="#007700">)<br />{<br />&nbsp;&nbsp;&nbsp;&nbsp;include(</font><font color="#0000BB">$class </font><font color="#007700">. </font><font color="#DD0000">'.php'</font><font color="#007700">);<br /><br />&nbsp;&nbsp;&nbsp;&nbsp;</font><font color="#FF8000">// Check to see if the include declared the class<br />&nbsp;&nbsp;&nbsp;&nbsp;</font><font color="#007700">if (!</font><font color="#0000BB">class_exists</font><font color="#007700">(</font><font color="#0000BB">$class</font><font color="#007700">, </font><font color="#0000BB">false</font><font color="#007700">)) {<br />&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</font><font color="#0000BB">trigger_error</font><font color="#007700">(</font><font color="#DD0000">"Unable to load class: $class"</font><font color="#007700">, </font><font color="#0000BB">E_USER_WARNING</font><font color="#007700">);<br />&nbsp;&nbsp;&nbsp;&nbsp;}<br />}<br /><br />if (</font><font color="#0000BB">class_exists</font><font color="#007700">(</font><font color="#DD0000">'MyClass'</font><font color="#007700">)) {<br />&nbsp;&nbsp;&nbsp;&nbsp;</font><font color="#0000BB">$myclass </font><font color="#007700">= new </font><font color="#0000BB">MyClass</font><font color="#007700">();<br />}<br /><br /></font><font color="#0000BB">?&gt;</font>
</font>
</code></TD
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>
  </P
><DIV
CLASS="note"
><BLOCKQUOTE
CLASS="note"
><P
><B
>注意: </B
>
    <CODE
CLASS="parameter"
>autoload</CODE
> 参数是 PHP 5 添加的。
   </P
></BLOCKQUOTE
></DIV
><P
>&#13;   参见 <A
HREF="function.interface-exists.html"
><B
CLASS="function"
>interface_exists()</B
></A
> 和 <A
HREF="function.get-declared-classes.html"
><B
CLASS="function"
>get_declared_classes()</B
></A
>。
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