问题
As many of you already know, PHP 5.4 alpha has been released. I have a question regarding the following.
Simplified string offset reading.
$str[1][0]is now a legal construct.
How exactly does $str[1][0] work?
EDIT: http://php.net/releases/NEWS_5_4_0_alpha1.txt
回答1:
This is a side effect, and was mentioned in the proposal here: http://php.markmail.org/thread/yiujwve6zdw37tpv
The feature is speed/optimization of string offsets.
Hi,
Recently I noticed that reading of string offset is performed in two steps. At first special string_offset variant of temporary_variable is created in zend_fetch_dimension_address_read() and then the real string value is created in _get_zval_ptr_var_string_offset().
I think we can create the real string in the first place. This makes 50% speed-up on string offset reading operation and allows to eliminate some checks and conditional brunches in VM.
The patch is attached (don't forget to regenerate zend_vm_execute.h to test it). However it changes behavior in one bogus case. The following code now will emit "b" (currently it generates a fatal error - cannot use string offset as an array).
$str = "abs"; var_dump($str[1][0]);I think it's not a problem at all. "b" makes sense because "abs"[1] -> "b" and "b"[0] -> "b".
I'm going to commit the patch in case of no objections.
Thanks. Dmitry.
回答2:
It just means that when reading a string offset PHP returns a string again, on which you again can access an offset. (And on that access yet another offset. It gets funny with $str[0][0][0][0][0][0])
Before PHP 5.4 you would get an "Cannot use string offset as an array" error.
回答3:
This can actually create some interesting bugs when you upgrade code from php 5.3 to 5.4.
In 5.3 this construct would return false:
$array = array("This is a string");
echo isset($array[0][0][0]);
In 5.4 this would return true.
来源：https://stackoverflow.com/questions/6588016/php-5-4s-simplified-string-offset-reading