Create Array without Array() function?

i was reading about how to create arrays without using function array()

and i saw this code :

$_="";
$_[+""]='';
echo $_;//output Array

and this :

$_="";
$_[+$_]++;
echo $_;//output Array

in the first code , why he put + before "" ?

in the second code why he put ++ after +$_ ?