print_r不理解输出的一部分

I run a print_r command on $_POST and I am getting the following response:

s:1289:"Array
(
    [id_master] => 12
    [id_controller] => 4
    [JSON] => {"uuid":"i~b1dad2c95c3efdc8521c84cd2891084289b9754b","relsno":102}
)";

Any idea what the s:1289: is?

This looks like a PHP serialized object. Here, s:1289 refers to the datatype, s for a string, and its length, which is 1289 characters.