How can conditional statements like if-else be effectively used in PHP to control the flow of actions based on user interactions with forms?

Conditional statements like if-else can be effectively used in PHP to control the flow of actions based on user interactions with forms by checking the values submitted through the form fields and executing different blocks of code accordingly. For example, you can use if-else statements to validate form inputs, process form data, or display different messages based on user input. 

```php
if(isset($_POST['submit'])) {
    $username = $_POST['username'];
    $password = $_POST['password'];

    if($username == 'admin' && $password == 'password') {
        echo 'Welcome, Admin!';
    } else {
        echo 'Invalid username or password. Please try again.';
    }
}
```

In this code snippet, we check if the form has been submitted by checking the existence of the 'submit' key in the $_POST array. We then retrieve the values of 'username' and 'password' fields submitted through the form. Finally, we use an if-else statement to check if the username and password match a predefined admin credentials and display appropriate messages based on the result.

Keywords

conditional statements if-else user interactions forms control flow

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