Rather than assuming we all know PHP, perhaps it would be better for you  
to explain the actual _behavior_ you're looking for.

That said, not knowing PHP myself I will take a wild guess and suppose  
that the code you show above declares an array that is indexed by a string  
("AccountNumber"), and which can associate three values with a single  
"AccountNumber" ("name", "City", and "balance").

In .NET a Hashtable or Dictionary<> would be a fine way to do that.  But  
you need to also declare a structure to hold the data first:

    struct MyData
    {
        public readonly string name;
        public readonly string City;
        public readonly decimal balance;

        public MyData(string name, string City, decimal balance)
        {
            this.name = name;
            this.City = City;
            this.balance = balance;
        }
    }

Then you can just add new instances of the struct to your dictionary:

    Dictionary<string, MyData> myArray = new Dictionary<string, MyData>();

    // Add stuff to the collection...
    myArray.Add("1234", new MyData("Foo", "Bar", 22));
    myArray.Add("2345", new MyData("Benny", "Hill", 22));
    // etc.

    // Print the "name" field of one of the elements in the collection...
    Console.WriteLine(myArray["2345"].name);

By the way, the code you tried would have actually worked, had you instead  
of clearing the hashtable just initialize the variable with a new one.  
Every time you called "myHashtable.Clear()", you removed everything from  
the Hashtable, and you only ever stored the one Hashtable instance in the  
Dictionary<> (in other words, all of your dictionary keys referenced the  
same Hashtable and you always emptied that one Hashtable after you added a  
reference to it to your dictionary).  It's not really the right way to  
manage the data structure anyway, but it would have worked had you not  
made that mistake.

The fact that you were having trouble suggests that you're not quite used  
to the idea of reference types.  To use C# effectively, you really need to  
understand that concept well.

Pete