It makes sense when you think of it in offsets. $string[offset] means that given $a = 'foo' then $a[0] is f and given $a = 'f' then $a[0] is f. Thus $a[0][0] would be the first offset of the first offset of "foo". $a[0][1] would not be valid, of course.
Also it's not really something you should ever use, but it's definitely more consistent than throwing an error.
It makes sense when you think of it in offsets. $string[offset] means that given $a = 'foo' then $a[0] is f and given $a = 'f' then $a[0] is f. Thus $a[0][0] would be the first offset of the first offset of "foo". $a[0][1] would not be valid, of course.
Also it's not really something you should ever use, but it's definitely more consistent than throwing an error.