PHP 5.4's simplified string offset reading

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.