curlies seem to work equivicantly here; but be careful using them inside strings…
<?php
$var = "I'm A String!";
echo $var[0]; //Outputs "I"
echo $var{0}; //Outputs "I"
echo "This is an $var[0] test of the $var{0} moo cow"; //Outputs "This is an I test of the I'm A String!{0} moo cow"
?>
It is “valid” in the sense of being supported by the language parser, as you can see it obviously works. However, it’s not recommended and certainly not the proper way to be specifying array items.