I have a JSON string that has 1-n numbers of lat/lng records. It looks something like this:
{\\\"lat\\\":37.790388261934424,\\\"lng\\\":-122.46047996826172},
In your case it's probably best to use:
$array = json_decode(stripslashes("[$input_string]"), 1);
Note that you will lose some precision on your float values in PHP.