In PHP, you can embed a variable into a string in these two ways:
echo "1: $a"; // outputs "1: b"
echo "2: {$a}"; // outputs "2: b"
However, you can also embed it in this arguably-more-confusing way:
And, moreover:
echo "4: ${$a}"; // outputs "4: c". This is insanity.
Luckily, these two cases are being deprecated and going away in PHP 9.