Answer
Solution:
Your Response
$str = "{'error': None, 'result': {'asks': [['7.5197', '13231.93']], 'bids': [['7.5101', '0.9']]}}";";
look like a json string, but isn't. With some substitutions it can be transformed into a valid json string.
$jsonStr = strtr($str, ["'" => '"', 'None' => '"None"']);
If there is already an array or an object in PHP, this steps can be omitted.
Now an object/array can be created with json_decode and the values can be accessed.
$obj = json_decode($jsonStr);
$valAsks = $obj->result->asks[0][0]; //"7.5197"
$valBids = $obj->result->bids[0][0]; //"7.5101"
