php-doc-en/language/oop5/autoload.xml

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 <sect1 xml:id="language.oop5.autoload" xmlns="http://docbook.org/ns/docbook">
  <title>Autoloading Objects</title>
  <para>
   Many developers writing object-oriented applications create
   one PHP source file per-class definition. One of the biggest
   annoyances is having to write a long list of needed includes
   at the beginning of each script (one for each class).
  </para>
  <para>
   In PHP 5, this is no longer necessary. You may define an
   __autoload function which is automatically
   called in case you are trying to use a class/interface which hasn't been
   defined yet. By calling this function the scripting engine is given
   a last chance to load the class before PHP fails with an error.
  </para>
  <note>
   <para>
    Exceptions thrown in __autoload function cannot be caught in the
    <link linkend="language.exceptions">catch</link> block and results in
    a fatal error.
   </para>
  </note>
  <note>
   <para>
    Autoloading is not available if using PHP in CLI
    <link linkend="features.commandline">interactive mode</link>.
   </para>
  </note>
  <note>
   <para>
    If the class name is used e.g. in <function>call_user_func</function> then
    it can contain some dangerous characters such as <literal>../</literal>.
    It is recommended to not use the user-input in such functions or at least
    verify the input in <function>__autoload</function>.
   </para>
  </note>
  <para>
   <example>
    <title>Autoload example</title>
    <para>
     This example attempts to load the classes <literal>MyClass1</literal>
     and <literal>MyClass2</literal> from the files <filename>MyClass1.php</filename>
     and <filename>MyClass2.php</filename> respectively.
    </para>
    <programlisting role="php">
<![CDATA[
<?php
function __autoload($class_name) {
    require_once $class_name . '.php';
}

$obj  = new MyClass1();
$obj2 = new MyClass2(); 
?>
]]>
    </programlisting>
   </example>
   <example>
    <title>Autoload other example</title>
    <para>
     This example attempts to load the interface <literal>ITest</literal>.
    </para>
    <programlisting role="php">
<![CDATA[
<?php

function __autoload($name) {
    var_dump($name);
}

class Foo implements ITest {
}

/*
string(5) "ITest"

Fatal error: Interface 'ITest' not found in ...
*/
?>
]]>
    </programlisting>
   </example>
  </para>

  <simplesect role="seealso">
   &reftitle.seealso;
   <para>
    <simplelist>
     <member><function>unserialize</function></member>
     <member><link linkend="unserialize-callback-func">unserialize_callback_func</link></member>
    </simplelist>
   </para>
  </simplesect>

 </sect1>
 
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