Consider the code:
$a = "foobar";
echo $a{3}; // prints b
I know $a[3] is 'b' but how come using {} in place of [] produec the same result ?
{} and [] are the same. it is your choice what to use. [] is more common though.
It's the same, but the {} syntax is deprecated as of PHP 5.3.
You can read here in official documentation about braces:
String s may also be accessed using braces, as in $str{42}, for the same purpose. However, this syntax is deprecated as of PHP 5.3.0. Use square brackets instead, such as $str[42].
EDIT This thread may interest you: "dropping curly braces"
It's just an alternative syntax; the two forms compile to exactly the same bytecode:
<?php
$str = "aaab";
echo $str{3};
echo $str[3];
number of ops: 9
compiled vars: !0 = $str
line # * op fetch ext return operands
---------------------------------------------------------------------------------
2 0 > EXT_STMT
1 ASSIGN !0, 'aaab'
3 2 EXT_STMT
3 FETCH_DIM_R $1 !0, 3
4 ECHO $1
4 5 EXT_STMT
6 FETCH_DIM_R $2 !0, 3
7 ECHO $2
8 > RETURN 1