I need to check if an option value from database is a json object.

This funtion following cannot work :

function isJson( string $str, $is_assoc = true ){
    json_decode( $str, $is_assoc );
    return (json_last_error() == JSON_ERROR_NONE);
}

It´s normal because with an SQL query I got :

a:2:{i:3;a:4:{s:5:"title";s:0:"";s:4:"text";s:503:"...}}

instead of

{ '3' : { 'title' : "", 'text' : "..." }...}}

How can I do to check if this data is a json object and to get this data as object ?

Final goal : Reaplace correclty value for json object in the database without breaking serialized data.