如何解码WordPress元数据数组

I'm using AWS S3 plugin for WordPress and trying to switch back manually. The plugin creates a new meta that encodes an array with curly brackets and string count. It looks like this:

[amazonS3_info] => Array ( [0] => 
 a:2:{s:6:"bucket";s:7:"test123";s:3:"key";s:56:"wp-content/uploads/2019/08/test.jpg"
;} )

Is this PHP or WordPress array? How do I encode and decode it?

Also, curious as to what's the point of the item and string count? Why not good old json?