从PHP数组读取时，[]的意思是什么？

Question

I've been writing a parser for PHP and while messing around with some third-party code, I run into a weird case regarding the [] operator.

Normally, [] is used without a key on the left side of an assignment meaning "add as the last element".

$a = array();
$a[] = 1;

will add 1 in the end of array $a.

In the aforementioned code though, I saw a line like this:

$r =& $a[];

When I tried to remove the &, I got a rather expected fatal error:

Fatal error: Cannot use [] for reading

With the & though, PHP does not complain at all. What is the meaning of the expression

& $a[];

?

Answer 1

The syntax means "use a reference to $a[]" or a reference to the newest created array element.

<?php
$a=array( );
$a[] = "a";
$a[] = "b";

print_r($a);

$r =& $a[];

print_r($a);

$r = "c";

print_r($a);

produces:

Array
(
    [0] => a
    [1] => b
)
Array
(
    [0] => a
    [1] => b
    [2] => 
)
Array
(
    [0] => a
    [1] => b
    [2] => c
)

Answer 2

My guess is that it adds an empty element to the array and gives $ra reference to that array element. Are you sure $a is supposed to be an array? It could also be trying to access a character of a string using the [] syntax, in which case I have no idea how that would work.