debug_zval_dump()

(PHP 4 >= 4.2.0, PHP 5, PHP 7)

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

The variable being evaluated.

返回值

没有返回值。

范例

Example #1debug_zval_dump()example

以上例程会输出：

&string(11) "Hello World" refcount(3)

Therefcountvalue returned by this function is non-obvious in certain circumstances. For example, a developer might expect the above example to indicate arefcountof2. The third reference is created when actually callingdebug_zval_dump().

This behavior is further compounded when a variable is not passed todebug_zval_dump()by reference. To illustrate, consider a slightly modified version of the above example:

以上例程会输出：

Whyrefcount(1)? Because a copy of$var1is being made, when the function is called.

This function becomes evenmoreconfusing when a variable with arefcountof1is passed (by copy/value):

以上例程会输出：

Arefcountof2, here, is extremely non-obvious. Especially considering the above examples. So what's happening?

When a variable has a single reference (as did$var1before it was used as an argument todebug_zval_dump()), PHP's engine optimizes the manner in which it is passed to a function. Internally, PHP treats$var1like a reference (in that therefcountis increased for the scope of this function), with the caveat thatifthe 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,ifdebug_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 therefcountto be incremented to2for 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)

"Fatal error: Call-time pass-by-reference has been removed ..." since PHP 5.something. So the is now invalid PHP code.

Actual Output debug_zval_dump()
example :1
$var1 = 'Hello World';
$var2 = '';
$var2 =& $var1;
debug_zval_dump("&$var1"); //output : string(12) "&Hello World" refcount(1) 
debug_zval_dump($var1);  //output : string(11) "Hello World" refcount(3)
debug_zval_dump(&$var1); //output : Parse error: syntax error, unexpected '&' i
example :2
$var1 = 'Hello World';
debug_zval_dump($var1);//string(11) "Hello World" refcount(3)
example : 3
$var1 = 'Hello World';
$var2 = 'who';
$var2 =& $var1;
debug_zval_dump($var1); //output ://string(11) "Hello World" refcount(3)

The add of "Call-time pass-by-reference" as E_DEPRECATED makes it impossible to get the real refcount without getting an error, since  and even  doesn't change anything.

If you're finding the interpretation of refcount confusing, the Xdebug extension offers a function similar to this one, but because the variable name is passed as a string, xdebug_debug_zval() doesn't scribble on the refcount with its own references to the zval.