//to safely serialize
$safe_string_to_store = base64_encode(serialize($multidimensional_array));

//to unserialize...
$array_restored_from_db = unserialize(base64_decode($encoded_serialized_string));

Errata

Use jsondecode and jsonencode instead of serialize and unserialize, use these functions as well as lighter, simplifies the use of these data in the frontend.

I had not thought of that thanks! @zaus @mbarzda the comment

json_encode($array);
json_decode($string);