An issue with the exclamation mark

Question

<?php
$a = 'hello';
$hello = "Hi";
$Hi = "hola";
echo $$a;
?>

// output is hola

<?php
$a = 'hello';
$hello = "Hi!";
$Hi! = "hola";
echo $$a;
?>

//output is ERROR!

what is wrong with the exclamation mark (!) ?

Answer 1

It outputs an error because an exclamation mark is a special character and cannot be used in a variable name.

Answer 2

mmm can we add the word Hi! inside parentheses? >> $(Hi!)="hola"

Answer 3

Including an exclamation mark in a variable name is not a legal declaration.

Answer 4

?