php - Decode a string from an array within an array back into an array

I have the following array (for instance)

Array
(
    [0] => 2
    [1] => 134
    [2] => 7
)
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Inside another array, of the form

Array
(
    [_page_list] => Array
        (
            [0] => a:3:{i:0;s:1:"2";i:1;s:3:"134";i:2;s:1:"7";}
        )
    [some more data...]
)
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This has been put into a Json and then decoded back again, giving me as an element just the string "a :3:{i:0;s:1:\"2\";i:1;s:3:\"134\";i:2;s:1:\"7\";}" inside the bigger array. This doesn't seem to be itself a json string, nor can it be simply cast back into an array.

How can I cast this string back into the proper array it used to be?