blocks|key|283190|text|它与任何接口无关，实际上非常简单。var_dump只显示那些已被其开发人员声明为的类属性，方法是调用如下C函数|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|283191|ZEND_API+int+zend_declare_property(...)
ZEND_API+int+zend_declare_property_null(...)
ZEND_API+int+zend_declare_property_bool(...)
ZEND_API+int+zend_declare_property_long(...)
ZEND_API+int+zend_declare_property_double(...)
ZEND_API+int+zend_declare_property_string(...)
ZEND_API+int+zend_declare_property_stringl(...)|code-block|syntax|javascript|283192|例如，类异常的属性在文件Zend/zend_exceptions.c中声明，如下所示|283193|+++zend_declare_property_string(default_exception_ce,+"message",+sizeof("message")-1,+"",+ZEND_ACC_PROTECTED+TSRMLS_CC);
+++zend_declare_property_string(default_exception_ce,+"string",+sizeof("string")-1,+"",+ZEND_ACC_PRIVATE+TSRMLS_CC);
+++zend_declare_property_long(default_exception_ce,+"code",+sizeof("code")-1,+0,+ZEND_ACC_PROTECTED+TSRMLS_CC);
+++zend_declare_property_null(default_exception_ce,+"file",+sizeof("file")-1,+ZEND_ACC_PROTECTED+TSRMLS_CC);
+++zend_declare_property_null(default_exception_ce,+"line",+sizeof("line")-1,+ZEND_ACC_PROTECTED+TSRMLS_CC);
+++zend_declare_property_null(default_exception_ce,+"trace",+sizeof("trace")-1,+ZEND_ACC_PRIVATE+TSRMLS_CC);
+++zend_declare_property_null(default_exception_ce,+"previous",+sizeof("previous")-1,+ZEND_ACC_PRIVATE+TSRMLS_CC);|283194|然后所有这些函数都会调用|283195|ZEND_API+int+zend_declare_property_ex(zend_class_entry+*ce,+const+char+*name,+...|283196|这将更新属性列表。然后是ext/standard/var.c中的var_dump，并通过调用php_object_property_dump来查找它们，后者通过相同的属性列表枚举它们。您可以看到有意公开的内部结构的。|CODE|283197|DOM扩展just+的开发人员选择不公开的类结构。它们根本不调用这些类型的函数。这就是你什么也看不见的原因。|283198|object(DOMDocument)#1+(0)+{
}|283199|如果你查看ext/dom/php_dom.c，你会发现一次属性声明。它是为DomException类准备的。它重新定义了属性code。|283200|zend_declare_property_long(dom_domexception_class_entry,+"code",+...|283201|如果异常转储看起来像|283202|var_dump+(new+Exception+('test',+102));

object(Exception)#1+(7)+{
++["message":protected]=>
++string(4)+"test"
++["string":"Exception":private]=>
++string(0)+""
++["code":protected]=>
++int(102)
++["file":protected]=>
++string(37)+"/usr/local/www/apache22/data/dump.php"
++["line":protected]=>
++int(3)
++["trace":"Exception":private]=>
++array(0)+{
++}
++["previous":"Exception":private]=>
++NULL
}|283203|DOMException转储有一点不同。|283204|+var_dump+(new+DOMException+());

object(DOMException)#2+(7)+{
++["message":protected]=>
++string(0)+""
++["string":"Exception":private]=>
++string(0)+""
++["file":protected]=>
++string(37)+"/usr/local/www/apache22/data/dump.php"
++["line":protected]=>
++int(9)
++["trace":"Exception":private]=>
++array(0)+{
++}
++["previous":"Exception":private]=>
++NULL
++["code"]=>
++int(0)
}|283205|看到code属性是如何移动到末尾的吗？这是因为重新声明。|283206|entityMap^0|U|A|0|0|0|0|0|0|C|I|W|8|1A|O|2Z|1|0|0|1I|0|0|0|1V|5|H|1Q|4|0|0|0|A|0|0|0|K|0|0|0|S|0^^$0|@$1|2|3|4|5|6|7|1F|8|@$9|1G|A|1H|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|1I|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|1J|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|1K|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|1L|8|@]|D|@]|E|$]]|$1|Q|3|R|5|H|7|1M|8|@]|D|@]|E|$I|J]]|$1|S|3|T|5|6|7|1N|8|@$9|1O|A|1P|B|U]|$9|1Q|A|1R|B|U]|$9|1S|A|1T|B|U]|$9|1U|A|1V|B|C]]|D|@]|E|$]]|$1|V|3|W|5|6|7|1W|8|@$9|1X|A|1Y|B|C]]|D|@]|E|$]]|$1|X|3|Y|5|H|7|1Z|8|@]|D|@]|E|$I|J]]|$1|Z|3|10|5|6|7|20|8|@$9|21|A|22|B|C]|$9|23|A|24|B|U]|$9|25|A|26|B|U]]|D|@]|E|$]]|$1|11|3|12|5|H|7|27|8|@]|D|@]|E|$I|J]]|$1|13|3|14|5|6|7|28|8|@$9|29|A|2A|B|C]]|D|@]|E|$]]|$1|15|3|16|5|H|7|2B|8|@]|D|@]|E|$I|J]]|$1|17|3|18|5|6|7|2C|8|@$9|2D|A|2E|B|C]]|D|@]|E|$]]|$1|19|3|1A|5|H|7|2F|8|@]|D|@]|E|$I|J]]|$1|1B|3|1C|5|6|7|2G|8|@$9|2H|A|2I|B|C]]|D|@]|E|$]]|$1|1D|3|-4|5|6|7|2J|8|@]|D|@]|E|$]]]|1E|$]]

It has nothing to do with any interfaces and in fact is very simple.
var_dump shows only those class properties that have been declared by their developers by calling such C-functions as 

<pre><code>ZEND_API int zend_declare_property(...)
ZEND_API int zend_declare_property_null(...)
ZEND_API int zend_declare_property_bool(...)
ZEND_API int zend_declare_property_long(...)
ZEND_API int zend_declare_property_double(...)
ZEND_API int zend_declare_property_string(...)
ZEND_API int zend_declare_property_stringl(...)
</code></pre>

For instance, the properties of the class Exception are declared in the file Zend/zend_exceptions.c like this

<pre><code> zend_declare_property_string(default_exception_ce, "message", sizeof("message")-1, "", ZEND_ACC_PROTECTED TSRMLS_CC);
 zend_declare_property_string(default_exception_ce, "string", sizeof("string")-1, "", ZEND_ACC_PRIVATE TSRMLS_CC);
 zend_declare_property_long(default_exception_ce, "code", sizeof("code")-1, 0, ZEND_ACC_PROTECTED TSRMLS_CC);
 zend_declare_property_null(default_exception_ce, "file", sizeof("file")-1, ZEND_ACC_PROTECTED TSRMLS_CC);
 zend_declare_property_null(default_exception_ce, "line", sizeof("line")-1, ZEND_ACC_PROTECTED TSRMLS_CC);
 zend_declare_property_null(default_exception_ce, "trace", sizeof("trace")-1, ZEND_ACC_PRIVATE TSRMLS_CC);
 zend_declare_property_null(default_exception_ce, "previous", sizeof("previous")-1, ZEND_ACC_PRIVATE TSRMLS_CC);
</code></pre>

All these functions then call 

<pre><code>ZEND_API int zend_declare_property_ex(zend_class_entry *ce, const char *name, ...
</code></pre>

which updates the property list. Then comes the <code>var_dump</code> in <code>ext/standard/var.c</code> and looks them up by calling <code>php_object_property_dump</code> which enumerates them through the same property list. You see the intentionally exposed inner structure.

The developers of the DOM extension just have chosen not to expose the structure of their classes. They simply do not call those sort of functions. That is why you see nothing.

<pre><code>object(DOMDocument)#1 (0) {
}
</code></pre>

If you look into <code>ext/dom/php_dom.c</code> you find a property declaration once. And it's for the DomException class. It redefines the property <code>code</code>. 

<pre><code>zend_declare_property_long(dom_domexception_class_entry, "code", ...
</code></pre>

If the Exception dump looks like

<pre><code>var_dump (new Exception ('test', 102));

object(Exception)#1 (7) {
 ["message":protected]=&gt;
 string(4) "test"
 ["string":"Exception":private]=&gt;
 string(0) ""
 ["code":protected]=&gt;
 int(102)
 ["file":protected]=&gt;
 string(37) "/usr/local/www/apache22/data/dump.php"
 ["line":protected]=&gt;
 int(3)
 ["trace":"Exception":private]=&gt;
 array(0) {
 }
 ["previous":"Exception":private]=&gt;
 NULL
}
</code></pre>

The DOMException dump is a little bit different. 

<pre><code> var_dump (new DOMException ());

object(DOMException)#2 (7) {
 ["message":protected]=&gt;
 string(0) ""
 ["string":"Exception":private]=&gt;
 string(0) ""
 ["file":protected]=&gt;
 string(37) "/usr/local/www/apache22/data/dump.php"
 ["line":protected]=&gt;
 int(9)
 ["trace":"Exception":private]=&gt;
 array(0) {
 }
 ["previous":"Exception":private]=&gt;
 NULL
 ["code"]=&gt;
 int(0)
}
</code></pre>

See how the code property moved to the end? It's because of redeclaration.

blocks|key|2995845|text|从PHP5.4.1开始，你终于可以var_dump+DOM对象了。请参阅https://gist.github.com/2499678|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2995846|2995847|这是一个bug：|2995848|2995849|https://bugs.php.net/bug.php?id=48527|unordered-list-item|2995850|2995851|entityMap|0|LINK|mutability|MUTABLE|url|https://gist.github.com/2499678|1^0|H|8|10|V|0|0|0|0|0|0|11|1|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]]|D|@$9|Z|A|10|1|11]]|E|$]]|$1|F|3|-4|5|6|7|12|8|@]|D|@]|E|$]]|$1|G|3|H|5|6|7|13|8|@]|D|@]|E|$]]|$1|I|3|-4|5|6|7|14|8|@]|D|@]|E|$]]|$1|J|3|K|5|L|7|15|8|@]|D|@$9|16|A|17|1|18]]|E|$]]|$1|M|3|-4|5|6|7|19|8|@]|D|@]|E|$]]|$1|N|3|-4|5|6|7|1A|8|@]|D|@]|E|$]]]|O|$P|$5|Q|R|S|E|$T|U]]|V|$5|Q|R|S|E|$T|K]]]]

Update: As of PHP 5.4.1 you can finally <code>var_dump</code> DOM objects. See <a href="https://gist.github.com/2499678" rel="nofollow noreferrer">https://gist.github.com/2499678</a>
<hr />
It's a bug:
<ul>
<li><a href="https://bugs.php.net/bug.php?id=48527" rel="nofollow noreferrer">https://bugs.php.net/bug.php?id=48527</a></li>
</ul>

Why doesn't <code>var_dump</code> work with <a href="https://php.net/domdocument" rel="nofollow noreferrer">DOMDocument</a> objects, while <code>print($dom-&gt;saveHTML())</code> produces output?

Why doesn't var_dump work with DOMDocument objects, while print($dom->saveHTML()) does?

为什么var_dump不能处理对象，而print($dom->saveHTML())可以产生输出？

问为什么var_dump不能处理DOMDocument对象，而print($dom->saveHTML())可以呢？
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问为什么var_dump不能处理DOMDocument对象，而print($dom->saveHTML())可以呢？
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