Yes. Strings can be seen as Char Arrays, and the way to access a position of an array is to use the [] operator, so there's no problem at all in using $str[0] (and I'm pretty sure is much faster than the substr method.
Have a nice day
Nicolás Hock Isaza
My only doubt would be how applicable this technique would be on multi-byte strings, but if that's not a consideration, then I suspect you're covered. (If in doubt, substr seems an obviously safe choice.)
However, from a big picture perspective, I have to wonder how often you need to access the 'n'th character in a string for this to be a key consideration.
I've used that notation before as well, with no ill side effects and no misunderstandings. It makes sense -- a string is just an array of characters, after all.
Speaking as a mere mortal, I would stick with $str[0]. As far as I'm concerned, it's quicker to grasp the meaning of $str[0] at a glance than substr($str, 0, 1). This probably boils down to a matter of preference.
As far as performance goes, well, profile profile profile. :) Or you could peer into the PHP source code...
The {} syntax is deprecated as of PHP 6. Square brackets are recommended.