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copied!<p>In PHP all values are saved in so called <code>zval</code>s. Those <code>zval</code>s contain the actual data, type information and - this is important for your question - a reference count. Have a look at the following snippet:</p> <pre><code>$a = new B; // $a points to zval(new B) with refcount=1 $b = $a; // $a, $b point to zval(new B) with refcount=2 (+1) $c = $b; // $a, $b, $c point to zval(new B) with refcount=3 (+1) unset($a); // $b, $c point to zval(new B) with refcount=2 (-1) </code></pre> <p>As soon as the <code>refcount</code> reaches <code>0</code> the <code>zval</code> is freed and the object destructor is called.</p> <p>Here are some examples of the <code>refcount</code> reaching <code>0</code>:</p> <ul> <li><p><code>unset</code>ing a variable:</p> <pre><code>$a = new B; // refcount=1 unset($a); // refcount=0 => __destruct! </code></pre> <p>But:</p> <pre><code>$a = new B; // refcount=1 $b = $a; // refcount=2 unset($a); // refcount=1 => no destruct as refcount > 0, even though unset() was called! </code></pre></li> <li><p>leaving function (or method) scope</p> <pre><code>function a() { $a = new B; // refcount=1 } // refcount=0 => __destruct! (as $a does not exist anymore) </code></pre></li> <li><p>script execution end</p> <pre><code>$a = new B; // refcount=1 die(); // refcount=0 => __destruct! (on script execution end all vars are freed) // doesn't need to be die(), can be just normal execution end </code></pre></li> </ul> <p>These obviously are not all conditions leading to a reduction of <code>refcount</code>, but the ones you will most commonly meet.</p> <p>Also I should mention that since PHP 5.3 circular references will be detected, too. So if object <code>$a</code> references object <code>$b</code> and <code>$b</code> references <code>$a</code> and there aren't any further references to <code>$a</code> or <code>$b</code> the <code>refcount</code>s of both will be <code>1</code>, but they still will be freed (and <code>__destruct</code>ed). In this case though the order of destruction is undefined behavior.</p>

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