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    <p><strong>Use the source, Luke</strong></p> <p>As usual, the answer lies in the source. PHP uses the following two functions internally to perform the increment and decrement operations:</p> <p><a href="http://lxr.php.net/xref/PHP_5_4/Zend/zend_operators.c#1794" rel="noreferrer"><code>ZEND_API int increment_function(zval *op1)</code></a></p> <p><a href="http://lxr.php.net/xref/PHP_5_4/Zend/zend_operators.c#1845" rel="noreferrer"><code>ZEND_API int decrement_function(zval *op1)</code></a></p> <p>These operations modify the <code>op1</code> argument based on its type (<code>NULL</code> is a type); inside <code>increment_function()</code> you can see the following branch <a href="http://lxr.php.net/xref/PHP_5_4/Zend/zend_operators.c#1809" rel="noreferrer">in the code</a>:</p> <pre class="lang-c prettyprint-override"><code>case IS_NULL: ZVAL_LONG(op1, 1); break; </code></pre> <p>The above code changes the type of <code>op1</code> into a number and sets its value to <code>1</code>.</p> <p>Conversely, <code>decrement_function()</code> offers no such branch and therefore the <a href="http://lxr.php.net/xref/PHP_5_4/Zend/zend_operators.c#1884" rel="noreferrer">default action</a> will be performed:</p> <pre class="lang-c prettyprint-override"><code>default: return FAILURE; </code></pre> <p>Running this code won't actually yield any observable failure, because the return values are absorbed in the Zend VM, but the variable definitely isn't updated either.</p> <p><strong>It's not a bug(tm)</strong></p> <p>You may be surprised to know that this behaviour, including that for boolean values, is actually <a href="http://php.net/manual/en/language.operators.increment.php" rel="noreferrer">documented</a>:</p> <blockquote> <p><strong>Note:</strong> The increment/decrement operators do not affect boolean values. Decrementing <code>NULL</code> values has no effect too, but incrementing them results in <code>1</code>.</p> </blockquote> <p>Regarding booleans:</p> <pre><code>$a = true; var_dump($a--); // true $a = false; var_dump($a++); // false </code></pre> <p>Regarding strings:</p> <pre><code>$letter = 'A'; var_dump(++$letter); // B var_dump(--$letter); // B </code></pre>
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    1. COIt is not a bug because it is documented?
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    2. CO@soc [What's a software bug ?](http://en.wikipedia.org/wiki/Software_bug) : A software bug is an error, flaw, failure, or fault in a computer program or system that produces an incorrect or **unexpected** result, or causes it to behave in **unintended** ways. But since it's documented then it's an **expected** and **intended** behaviour.
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    3. CO@soc The slight irony in that part may have fallen on blind eyes, so I've made a slight edit. That said, I'm surprised to see you drawn to this question, given your [earlier](http://stackoverflow.com/questions/2968913/which-language-to-choose-for-a-live-web-application/2969493#2969493) [contributions](http://stackoverflow.com/a/3922387/1338292) to this tag ;-)
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