Double quoted nested array won't work - php

Question

Consider this :
$a = 'u';
$b[$a] = 'v';
$c[$b[$a]] = 'w';
This works fine:
php > echo $c[$b[$a]];
w
This also works:
php > echo '$c[$b[$a]]';
'$c[$b[$a]]'
However this leads to a syntax error:
php > echo "$c[$b[$a]]";
PHP Parse error: syntax error, unexpected '[', expecting ']'
While a shorter version works:
php > echo "$b[$a]";
v
Why?

Answer 1

In a double quoted string there are two kinds of variable parsing, simple and complex. Simple parsing will only work with a single dimensional array or object. So these will work:
echo "Valid: $foo[1]";
echo "Valid: $foo[bar]";
echo "Valid: $foo->bar";
But these will not:
echo "Invalid: $foo[1][2]";
echo "Invalid: $foo->bar->baz";
echo "Invalid: $foo->bar()[2]";
Complex parsing is what halfer suggested in his answer, which wraps the expression in curly braces, and will indeed solve your problem:
echo "Valid: ${foo[1][2]}";
echo "Valid: ${foo->bar->baz}";
echo "Valid: ${foo->bar()[2]}";

Answer 2

The short answer is: don't write PHP like this! If it is confusing to read then use an intermediate variable.
However, it can be fixed: it looks like PHP just wants the brace delimiters at the outermost edges of the variable:
<?php
$a = 'u';
$b[$a] = 'v';
$c[$b[$a]] = 'w';
echo "{$c[$b[$a]]}";
This is also fine as well:
echo "${c[$b[$a]]}";
In the first case, the whole thing is wrapped in braces, and in the second, the $ is allowed outside of the brace construct to say that the next bit is a variable.