php-doc-en/reference/var/functions/debug-zval-dump.xml

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<refentry xml:id="function.debug-zval-dump" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
 <refnamediv>
  <refname>debug_zval_dump</refname>
  <refpurpose>Dumps a string representation of an internal zend value to output</refpurpose>
 </refnamediv>
 <refsect1 role="description">
  &reftitle.description;
  <methodsynopsis>
   <type>void</type><methodname>debug_zval_dump</methodname>
   <methodparam><type>mixed</type><parameter>variable</parameter></methodparam>
  </methodsynopsis>
  <para>
   Dumps a string representation of an internal zend value to output.
  </para>
 </refsect1>
 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>variable</parameter></term>
     <listitem>
      <para>
       The variable being evaluated.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
 </refsect1>
 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   &return.void;
  </para>
 </refsect1>
 <refsect1 role="examples">
  &reftitle.examples;
  <para>
   <example>
    <title><function>debug_zval_dump</function> example</title>
    <programlisting role="php">
<![CDATA[
<?php
$var1 = 'Hello World';
$var2 = '';

$var2 =& $var1;

debug_zval_dump(&$var1);
?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
&string(11) "Hello World" refcount(3)
]]>
    </screen>
   </example>
  </para>
  <note>
   <title>Beware the <literal>refcount</literal></title>
   <para>
    The <literal>refcount</literal> value returned by this function is
    non-obvious in certain circumstances. For example, a developer might
    expect the above example to indicate a <literal>refcount</literal> of
    <literal>2</literal>. The third reference is created when actually
    calling <function>debug_zval_dump</function>.
   </para>
   <para>
    This behavior is further compounded when a variable is not passed to
    <function>debug_zval_dump</function> by reference. To illustrate, consider
    a slightly modified version of the above example:
   </para>
   <para>
    <example>
     <title/>
     <programlisting role="php">
<![CDATA[
<?php
$var1 = 'Hello World';
$var2 = '';

$var2 =& $var1;

debug_zval_dump($var1); // not passed by reference, this time
?>
]]>
     </programlisting>
     &example.outputs;
     <screen>
<![CDATA[
string(11) "Hello World" refcount(1)
]]>
     </screen>
    </example>
   </para>
   <para>
    Why <literal>refcount(1)</literal>? Because a copy of <literal>$var1</literal> is
    being made, when the function is called.
   </para>
   <para>
    This function becomes even <emphasis>more</emphasis> confusing when a
    variable with a <literal>refcount</literal> of <literal>1</literal> is
    passed (by copy/value):
   </para>
   <para>
    <example>
     <title/>
     <programlisting role="php">
<![CDATA[
<?php
$var1 = 'Hello World';

debug_zval_dump($var1);
?>
]]>
     </programlisting>
     &example.outputs;
     <screen>
<![CDATA[
string(11) "Hello World" refcount(2)
]]>
     </screen>
    </example>
   </para>
   <para>
    A <literal>refcount</literal> of <literal>2</literal>, here, is extremely
    non-obvious. Especially considering the above examples. So what's
    happening?
   </para>
   <para>
    When a variable has a single reference (as did <literal>$var1</literal>
    before it was used as an argument to <function>debug_zval_dump</function>),
    PHP's engine optimizes the manner in which it is passed to a function.
    Internally, PHP treats <literal>$var1</literal> like a reference (in that
    the <literal>refcount</literal> is increased for the scope of this
    function), with the caveat that <emphasis>if</emphasis> the passed reference
    happens to be written to, a copy is made, but only at the moment of
    writing. This is known as "copy on write."
   </para>
   <para>
    So, if <function>debug_zval_dump</function> happened to write to its sole
    parameter (and it doesn't), then a copy would be made. Until then, the
    parameter remains a reference, causing the <literal>refcount</literal> to
    be incremented to <literal>2</literal> for the scope of the function call.
   </para>
  </note>
 </refsect1>
 <refsect1 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member><function>var_dump</function></member>
    <member><function>debug_backtrace</function></member>
    <member><link linkend="language.references">References Explained</link></member>
    <member><link xlink:href="&url.derick.references;">References Explained (by Derick Rethans)</link></member>
   </simplelist>
  </para>
 </refsect1>
</refentry>

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