Entity JSON encode and manually decode

In entity

Code:

protected $casts = [
    'courses'        => 'array',
    'courses_object' => 'json',
    'courses_array'  => 'json-array',
];

The posted array

Code:

courses = array{"bsc-bio", "bsc-bed", "bca"}

It inserted in database as

Code:

"a:3:{i:0;s:7:"bsc-bio";i:1;s:7:"bsc-bed";i:2;s:3:"bca";}"

How to manually decode it?

Code:

json_decode('a:3:{i:0;s:7:"bsc-bio";i:1;s:7:"bsc-bed";i:2;s:3:"bca";}')

json_decode() returns NULL.