In PHP form handling, what is the significance of checking for the existence of a button value before processing form data?

Checking for the existence of a button value before processing form data is important to ensure that the form submission was triggered by the intended button click. This helps prevent processing form data if the form was submitted by other means, such as a script or direct URL access. By validating the button value, you can ensure that the form data is only processed when the form is submitted through the expected user interaction.

```php
if(isset($_POST['submit_button'])) {
    // Process form data here
}
```
In this code snippet, we check if the 'submit_button' value is set in the $_POST superglobal array before processing the form data. This ensures that the form data is only processed when the submit button is clicked.