PHP: using variable value as variable name: curious about syntax issue

Question

Suppose I have the following:

$foo = "bar";
$bar = "hello";

Then the string "hello" can be echoed to standard output as either:

echo $$foo;

or

echo ${$foo};

I was wondering what the difference is between these two statements in general. That is, what is the purpose of the braces around the evaluated variable name in the second syntax displayed above?

Answer 1

The only reason to use ${$foo} is to be able to combine $foo to something else, for example:

$idx = 4;
$bar3 = 20;
$bar4 = 7;
echo ${$foo.$idx}

This would return the value in $bar4 since $idx is worth 4;