php-doc-en/reference/weakref/book.xml

<?xml version="1.0" encoding="utf-8"?>
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<book xml:id="book.weakref" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
 <title>Weak References</title>
 <titleabbrev>Weakref</titleabbrev>
 
 <preface xml:id="intro.weakref">
  &reftitle.intro;
  <para>
   Weak references provide a non-intrusive gateway to ephemeral objects. Unlike
   normal (strong) references, weak references do not prevent the garbage
   collector from freeing that object. For this reason, an object may be
   destroyed even though a weak reference to that object still exists. In such
   conditions, the weak reference seamlessly becomes invalid.
  </para>

  <para>
   <example>
    <title><classname>Weakref</classname> usage example</title>
    <programlisting role="php">
<![CDATA[
<?php
class MyClass {
    public function __destruct() {
        echo "Destroying object!\n";
    }
}

$o1 = new MyClass;

$r1 = new Weakref($o1);

if ($r1->valid()) {
    echo "Object still exists!\n";
    var_dump($r1->get());
} else {
    echo "Object is dead!\n";
}

unset($o1);

if ($r1->valid()) {
    echo "Object still exists!\n";
    var_dump($r1->get());
} else {
    echo "Object is dead!\n";
}
?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
Object still exists!
object(MyClass)#1 (0) {
}
Destroying object!
Object is dead!
]]>
     </screen>
    </example>
   </para>
 </preface>

 &reference.weakref.setup;
 &reference.weakref.weakref;
 &reference.weakref.weakmap;

</book>

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