如何在Codeigniter中的字符串的最后一个索引中添加/替换字符？

I have a data like this:

"[-8.390865416667355, 115.23490905761719],[-8.483238563913513, 115.19783020019531],[-8.504970203442133, 115.40382385253906]"

I want to add character "[" in the first index of my string value and "]" in my last index. But i dont know how to insert the character in the last index of my string value so my string would be appear like this:

"[[-8.390865416667355, 115.23490905761719],[-8.483238563913513, 115.19783020019531],[-8.504970203442133, 115.40382385253906]]"

You could just concatenate, like so:

$str = "[-8.390865416667355, 115.23490905761719],[-8.483238563913513, 115.19783020019531],[-8.504970203442133, 115.40382385253906]";

$new_str = "[".$str."]";

echo $new_str;