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171 lines
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HTML
171 lines
8.7 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<head>
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<meta http-equiv="content-type" content="text/html; charset=UTF-8">
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<title>Dumps a string representation of an internal zend value to output</title>
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</head>
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<body class="docs"><div id="layout">
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<div id="layout-content"><div id="function.debug-zval-dump" class="refentry">
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<div class="refnamediv">
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<h1 class="refname">debug_zval_dump</h1>
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<p class="verinfo">(PHP 4 >= 4.2.0, PHP 5, PHP 7)</p><p class="refpurpose"><span class="refname">debug_zval_dump</span> — <span class="dc-title">Dumps a string representation of an internal zend value to output</span></p>
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</div>
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<div class="refsect1 description" id="refsect1-function.debug-zval-dump-description">
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<h3 class="title">说明</h3>
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<div class="methodsynopsis dc-description">
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<span class="methodname"><strong>debug_zval_dump</strong></span>
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( <span class="methodparam"><span class="type"><a href="language.pseudo-types.html#language.types.mixed" class="type mixed">mixed</a></span> <code class="parameter">$variable</code></span>
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[, <span class="methodparam"><span class="type"><a href="language.pseudo-types.html#language.types.mixed" class="type mixed">mixed</a></span> <code class="parameter">$...</code></span>
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] ) : <span class="type"><span class="type void">void</span></span></div>
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<p class="para rdfs-comment">
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Dumps a string representation of an internal zend value to output.
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</p>
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</div>
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<div class="refsect1 parameters" id="refsect1-function.debug-zval-dump-parameters">
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<h3 class="title">参数</h3>
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<p class="para">
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<dl>
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<dt>
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<code class="parameter">variable</code></dt>
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<dd>
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<p class="para">
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The variable being evaluated.
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</p>
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</dd>
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</dl>
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</p>
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</div>
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<div class="refsect1 returnvalues" id="refsect1-function.debug-zval-dump-returnvalues">
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<h3 class="title">返回值</h3>
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<p class="para">
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没有返回值。
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</p>
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</div>
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<div class="refsect1 examples" id="refsect1-function.debug-zval-dump-examples">
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<h3 class="title">范例</h3>
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<p class="para">
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<div class="example" id="example-6355">
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<p><strong>Example #1 <span class="function"><strong>debug_zval_dump()</strong></span> example</strong></p>
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<div class="example-contents">
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<div class="phpcode"><pre><span style="color: #000000">
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<span style="color: #0000BB"><?php<br />$var1 </span><span style="color: #007700">= </span><span style="color: #DD0000">'Hello World'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$var2 </span><span style="color: #007700">= </span><span style="color: #DD0000">''</span><span style="color: #007700">;<br /><br /></span><span style="color: #0000BB">$var2 </span><span style="color: #007700">=& </span><span style="color: #0000BB">$var1</span><span style="color: #007700">;<br /><br /></span><span style="color: #0000BB">debug_zval_dump</span><span style="color: #007700">(&</span><span style="color: #0000BB">$var1</span><span style="color: #007700">);<br /></span><span style="color: #0000BB">?></span>
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</span>
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</pre></div>
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</div>
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<div class="example-contents"><p>以上例程会输出：</p></div>
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<div class="example-contents screen">
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<div class="cdata"><pre>
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&string(11) "Hello World" refcount(3)
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</pre></div>
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</div>
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</div>
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</p>
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<blockquote class="note"><p><strong class="note">Note</strong>:
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<strong>Beware the <em>refcount</em></strong><br />
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<p class="para">
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The <em>refcount</em> value returned by this function is
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non-obvious in certain circumstances. For example, a developer might
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expect the above example to indicate a <em>refcount</em> of
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<em>2</em>. The third reference is created when actually
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calling <span class="function"><strong>debug_zval_dump()</strong></span>.
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</p>
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<p class="para">
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This behavior is further compounded when a variable is not passed to
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<span class="function"><strong>debug_zval_dump()</strong></span> by reference. To illustrate, consider
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a slightly modified version of the above example:
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</p>
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<p class="para">
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<div class="example" id="example-6356">
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<div class="example-contents">
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<div class="phpcode"><pre><span style="color: #000000">
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<span style="color: #0000BB"><?php<br />$var1 </span><span style="color: #007700">= </span><span style="color: #DD0000">'Hello World'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$var2 </span><span style="color: #007700">= </span><span style="color: #DD0000">''</span><span style="color: #007700">;<br /><br /></span><span style="color: #0000BB">$var2 </span><span style="color: #007700">=& </span><span style="color: #0000BB">$var1</span><span style="color: #007700">;<br /><br /></span><span style="color: #0000BB">debug_zval_dump</span><span style="color: #007700">(</span><span style="color: #0000BB">$var1</span><span style="color: #007700">); </span><span style="color: #FF8000">// not passed by reference, this time<br /></span><span style="color: #0000BB">?></span>
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</span>
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</pre></div>
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</div>
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<div class="example-contents"><p>以上例程会输出：</p></div>
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<div class="example-contents screen">
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<div class="cdata"><pre>
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string(11) "Hello World" refcount(1)
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</pre></div>
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</div>
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</div>
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</p>
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<p class="para">
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Why <em>refcount(1)</em>? Because a copy of <em>$var1</em> is
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being made, when the function is called.
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</p>
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<p class="para">
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This function becomes even <em class="emphasis">more</em> confusing when a
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variable with a <em>refcount</em> of <em>1</em> is
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passed (by copy/value):
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</p>
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<p class="para">
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<div class="example" id="example-6357">
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<div class="example-contents">
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<div class="phpcode"><pre><span style="color: #000000">
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<span style="color: #0000BB"><?php<br />$var1 </span><span style="color: #007700">= </span><span style="color: #DD0000">'Hello World'</span><span style="color: #007700">;<br /><br /></span><span style="color: #0000BB">debug_zval_dump</span><span style="color: #007700">(</span><span style="color: #0000BB">$var1</span><span style="color: #007700">);<br /></span><span style="color: #0000BB">?></span>
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</span>
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</pre></div>
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</div>
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<div class="example-contents"><p>以上例程会输出：</p></div>
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<div class="example-contents screen">
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<div class="cdata"><pre>
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string(11) "Hello World" refcount(2)
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</pre></div>
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</div>
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</div>
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</p>
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<p class="para">
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A <em>refcount</em> of <em>2</em>, here, is extremely
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non-obvious. Especially considering the above examples. So what's
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happening?
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</p>
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<p class="para">
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When a variable has a single reference (as did <em>$var1</em>
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before it was used as an argument to <span class="function"><strong>debug_zval_dump()</strong></span>),
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PHP's engine optimizes the manner in which it is passed to a function.
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Internally, PHP treats <em>$var1</em> like a reference (in that
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the <em>refcount</em> is increased for the scope of this
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function), with the caveat that <em class="emphasis">if</em> the passed reference
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happens to be written to, a copy is made, but only at the moment of
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writing. This is known as "copy on write."
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</p>
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<p class="para">
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So, if <span class="function"><strong>debug_zval_dump()</strong></span> happened to write to its sole
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parameter (and it doesn't), then a copy would be made. Until then, the
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parameter remains a reference, causing the <em>refcount</em> to
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be incremented to <em>2</em> for the scope of the function call.
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</p>
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</p></blockquote>
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</div>
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<div class="refsect1 seealso" id="refsect1-function.debug-zval-dump-seealso">
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<h3 class="title">参见</h3>
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<p class="para">
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<ul class="simplelist">
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<li class="member"><span class="function"><a href="var_dump.html" class="function" rel="rdfs-seeAlso">var_dump()</a> - 打印变量的相关信息</span></li>
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<li class="member"><span class="function"><a href="debug_backtrace.html" class="function" rel="rdfs-seeAlso">debug_backtrace()</a> - 产生一条回溯跟踪(backtrace)</span></li>
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<li class="member"><a href="language.references.html" class="link">References Explained</a></li>
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<li class="member"><a href="http://derickrethans.nl/php_references_article.php" class="link external">» References Explained (by Derick Rethans)</a></li>
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</ul>
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</p>
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</div>
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</div></div></div></body></html> |