The $_POST Array

We mentioned in an earlier topics that a browser sends user input through either a GET request or a POST request. The POST request is usually preferred, and we use it here. The web server bundles up all the user input (even if the form was filled out with a hundred fields) and puts it into an array named $_POST.
$_POST is an associative array.Depending on whether a form has been set to use the POST or the GET method, either the $_POST or the $_GET associative array will be populated with the form data. They can both be read in exactly the same way.
Each field has an element in the array named after that field. So if a form contained a field named isbn, the $_POST array contains an element keyed by the word isbn. The PHP program can read that field by referring to either $_POST[‘isbn’] or $_POST[“isbn”] (single and double quotes have the same effect in this case).
If the $_POST syntax still seems complex to you, rest assured that you can just use the convention I’ve shown in previous example, copy the user’s input to other variables, and forget about $_POST after that. This is normal in PHP programs: they retrieve all the fields from $_POST at the beginning of the program and then ignore it.

There is no reason to write to an element in the $_POST array. Its only purpose is to communicate information from the browser to the program, and you’re better off copying data to your own variables before altering it.