Php/docs/function.debug-zval-dump
debug_zval_dump
(PHP 4 >= 4.2.0, PHP 5, PHP 7)
debug_zval_dump — Dumps a string representation of an internal zend value to output
说明
Dumps a string representation of an internal zend value to output.
参数
variable
- The variable to dump.
variables
- Further variables to dump.
返回值
没有返回值。
范例
Example #1 debug_zval_dump() example
<?php$var1 = 'Hello World';$var2 = ;$var2 =& $var1;debug_zval_dump(&$var1);?>
以上例程会输出:
&string(11) "Hello World" refcount(3)
Note:
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 arefcount
of2
. 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?>
以上例程会输出:
string(11) "Hello World" refcount(1)Why
refcount(1)
? Because a copy of$var1
is being made, when the function is called.This function becomes even more confusing when a variable with a
refcount
of1
is passed (by copy/value):
<?php$var1 = 'Hello World';debug_zval_dump($var1);?>
以上例程会输出:
string(11) "Hello World" refcount(2)A
refcount
of2
, 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 therefcount
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 to2
for the scope of the function call.
参见
- var_dump() - 打印变量的相关信息
- debug_backtrace() - 产生一条回溯跟踪(backtrace)
- References Explained
- » References Explained (by Derick Rethans)