How to call a function and return data from a function to the global scope?

I created an HTML form that, on submit, opens submit_form.php. Inside, I process the data entered into the form and send a confirmation email. I wrote a function for the mail-sending-process.
Inside the .php file, I have PHP, as well as HTML code. The important parts of the file look like this:

<?php
    $success = "undefined";

    /* ... */

    function send_mail($to, $from, $fromName, $subject, $message) {
        /* headers, etc. */
    
        if (mail($to, $subject, "", $header)) {
            $success = "true";
        } else {
            $success = "false";
        }
    }
?>

<html>
    <body>
        success? <?php echo $success; ?>
    </body
</html>

Now, when loading the HTML on the success site (HTML code below the .php script, in the same file), I put this code to verify whether the email was sent successfully:

success? <?php echo $success; ?>

...which presents the following output: success? undefined

Solution

The problem is that the $success variable is not in global scope. Use global to make it global.

function send_mail($to, $from, $fromName, $subject, $message) {
        global $success;

        /* headers, etc. */

        if (mail($to, $subject, "", $header)) {
            $success = "true";
        } else {
            $success = "false";
        }
    }

Variable scope means where the variable is visible. In php,variables local to a function don't live in the global scope by default.