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<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision: 297617 $ -->
<chapter xml:id="internals2.variables" xmlns="http://docbook.org/ns/docbook">
<title>Working with variables</title>
<section xml:id="internals2.variables.intro">
<title>Intro</title>
<para>
For working with variables in PHP's core you have to learn about different
fundamental concepts used in PHP. Firstly PHP is a dynamic and weak typed
language. Secondly PHP uses a copy on write mechanism with reference
counting for memory handling. Please check the <xref
linkend="features.gc.refcounting-basics"/> chapter for details how
reference counting and references work.
</para>
<para>
PHP variables, in general, consist out of two things: The label, which
might, for instance, be an entry in a symbol table, and the actual variable
container. For the most parts of this manual we will focus on the variable
container.
</para>
<para>
The variable container, in code called <code>zval</code>, is holding all
data needed to handle the variable. This includes not only the actual value
but also the current type, a counter counting the number of labels pointing
to this container and a flag whether these labels should be treated as
references or copies. In PHP 5.3 the relevant structures, which you can
find in <code>Zend/zend.h</code>, look like this:
</para>
<screen>
<![CDATA[
typedef struct _zval_struct zval;
typedef union _zvalue_value {
long lval; /* long value */
double dval; /* double value */
struct { /* string type */
char *val;
int len;
} str;
HashTable *ht; /* hash table value */
zend_object_value obj;
} zvalue_value;
struct _zval_struct {
/* Variable information */
zvalue_value value; /* value */
zend_uint refcount__gc;
zend_uchar type; /* active type */
zend_uchar is_ref__gc;
};
]]>
</screen>
<para>
In the <code>zvalue_value</code> one can find the internal representation
for the different types the fields used should be clear from the names and
comments - especially if one knows that PHP's arrays are infact hash
tables. Nonetheless, knowing PHP's types one might miss a few:
<code>NULL</code>, <code>boolean</code> and <code>resources</code>. For
<code>NULL</code> we need no value, as <code>NULL</code> is the value of
that type. For <code>boolean</code> and <code>resource</code> values PHP
re-uses the value field. In the case of a <code>boolean</code> it holds
either <code>0</code> for <code>false</code> or <code>1</code> for
<code>true</code>. For <code>resource</code>-typed variables it holds the
resource id.
</para>
<para>
Now the good message is that you don't have to know these things in detail
as there are - like always in PHP - acces macros. The bad news is that there
are many of them: There are macros to access any aspect of the
<code>zval</code> and then, as one often deals with pointers to
<code>zval</code>s and even pointers to pointers to <code>zval</code>s, for
most of them there are shortcuts dereferencing these pointers. These macros
are spread over <code>Zend/zend.h</code>,
<code>Zend/zend_operators.h</code> and <code>Zend/zend_API.h</code>.
</para>
</section>
<section xml:id="internals2.variables.creating">
<title>Creating variables and setting values</title>
<para>
<!-- ... -->
</para>
</section>
</chapter>
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