Why are assignment operators (=) invalid in a foreach loop?

Why are assignment operators (=) invalid in a foreach loop? I'm using C#, but I would assume that the argument is the same for other languages that support foreach (e.g. PHP). For example, if I do something like this:

string[] sArray = new string[5];

foreach (string item in sArray)
{
   item = "Some assignment.\r\n";
}

I get an error, "Cannot assign to 'item' because it is a 'foreach iteration variable'."