Nota:
Understanding the refcount
The refcount
value shown by this function may be
surprising without a detailed understanding of the engine's implementation.
The Zend Engine uses reference counting for two different purposes:
-
Optimizing memory usage using a technique called "copy on write",
where multiple variables holding the same value point to the same copy
in memory. When any of the variables is modified, it is pointed to a new
copy in memory, and the reference count on the original is decreased by 1.
-
Tracking variables which have been assigned or passed by reference (see
References Explained). This
refcount is stored on a separate reference zval, pointing to the zval
for the current value. This additional zval is not currently shown by
debug_zval_dump().
Because debug_zval_dump() takes its input as normal
parameters, passed by value, the copy on write technique will be used
to pass them: rather than copying the data, the refcount will be increased
by one for the lifetime of the function call. If the function modified the
parameter after receiving it, then a copy would be made; since it does not,
it will show a refcount one higher than in the calling scope.
The parameter passing also prevents debug_zval_dump()
showing variables which have been assigned by reference. To illustrate,
consider a slightly modified version of the above example:
<?php
$var1 = 'Hello';
$var1 .= ' World';
// Point three variables as references to the same value
$var2 =& $var1;
$var3 =& $var1;
debug_zval_dump($var1);
?>
Il precedente esempio visualizzerà:
string(11) "Hello World" refcount(2)
Although $var1, $var2, and
$var3 are linked as references, only the
value is passed to debug_zval_dump().
That value is used once by the set of references, and once inside the
debug_zval_dump(), so shows a refcount of 2.
Further complications arise because of optimisations made in the engine for
different data types. Some types such as integers do not use "copy on write",
so do not show a refcount at all. In other cases, the refcount shows extra
copies used internally, such as when a literal string or array is stored as
part of a code instruction.