Very well made. I'll make sure to link random people to these whenever it's relevant.
For "operators and variables", one interesting thing non-existant arrays can do that you can't do normally is use variables with spaces or accented letters:
$foo["two words"] = "hello";
==> echo($foo["two words"]);
hello
==> echo($footwo words);
Syntax error in input
Whether that's too far for a basic introduction is up to you, though.
Also the example says echo($a @ $b); for the "NL" operator.