php - $_REQUEST を理解する。これが $_REQUEST['submit'] を取得しないのはなぜですか?

Question

以下のコードは、DB から行を削除できますが、挿入できません。何故ですか？

<?php

    mysql_connect("localhost","formuser","password") or die(mysql_error());
    mysql_select_db("formpractice") or die(mysql_error());


    $name = $_POST['name'];
    $address = $_POST['address'];

    // Add into DB Query
    $insert = "INSERT INTO Users(name, address) VALUES ('$name', '$address')";
    $insert_result = mysql_query($insert);

    // Delete from DB Query
    $delete = "DELETE FROM Users WHERE name = '$name' OR address = '$address'";
    $delete_result = mysql_query($delete);

    if(isset($_REQUEST['submit'])) {
        if ($insert_result) {
            echo "Success";
        } else {
            echo "Failed to submit";
        }
        echo 'Submit Request'; 

    } else if (isset($_REQUEST['delete'])) {
        if ($delete_result) {
            echo "Deleted";
        } else {
            echo "Failed to submit";
        }
    }


?>