why ${0} is valid but ${0 is not valid in PHP

Question

Why ${0} is valid but ${0 is not valid? it means that curly braces evaluate what's inside them and that's why we
cannot have ${0?

Variables shouldn't start from 0 so it means that it started from braces? if yes why I cannot have ${0? and what is the variable here finally? $0 is what it interprets? if yes so it is not a legal variable.

Answer 1

The variable ${0} is a variable variable. For instance:

$foo = 50;
$fee = 'foo';
echo ${$fee}."n"; // This will output 50.
echo ${'foo'}."n"; // Again this will out 50.

To leave off the closing curly brace is to leave the expression unterminated. Similar to leaving off a parenthesis, square brace, or conditional brace. The engine needs to know the end of the variable to which it should retrieve it's value to then find the actual variable to look up in the current context.