Notă:
Beware the refcount
The refcount
value returned by this function is
non-obvious in certain circumstances. For example, a developer might
expect the above example to indicate a refcount
of
2
. The third reference is created when actually
calling debug_zval_dump().
This behavior is further compounded when a variable is not passed to
debug_zval_dump() by reference. To illustrate, consider
a slightly modified version of the above example:
<?php
$var1 = 'Hello World';
$var2 = '';
$var2 =& $var1;
debug_zval_dump($var1); // not passed by reference, this time
?>
Exemplul de mai sus va afișa:
string(11) "Hello World" refcount(1)
Why refcount(1)
? Because a copy of $var1
is
being made, when the function is called.
This function becomes even more confusing when a
variable with a refcount
of 1
is
passed (by copy/value):
<?php
$var1 = 'Hello World';
debug_zval_dump($var1);
?>
Exemplul de mai sus va afișa:
string(11) "Hello World" refcount(2)
A refcount
of 2
, here, is extremely
non-obvious. Especially considering the above examples. So what's
happening?
When a variable has a single reference (as did $var1
before it was used as an argument to debug_zval_dump()),
PHP's engine optimizes the manner in which it is passed to a function.
Internally, PHP treats $var1
like a reference (in that
the refcount
is increased for the scope of this
function), with the caveat that if 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."
So, if debug_zval_dump() 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 refcount
to
be incremented to 2
for the scope of the function call.