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I posted four suggestions earlier,
They were:
@
&
^
My favourites are the asterisk or caret.
That’s no different than @ being invalid because it’s already in use.
$ php -a
Interactive shell
php > define('a', 1);
php > define('b', 2);
php > echo @a . "\n";
1
php > echo (a&b) . "\n";
0
php > echo (a*b) . "\n";
2
php > echo (a^b) . "\n";
3
php >
However: ☃ has a long history of being available to PHP.
S