Simple PHP code that generates a license key and displays it in the frontend.

Here is an example of a simple PHP code that generates a license key and displays it in the frontend:

<?php

// Function to generate a license key
function generate_license_key() {
    $alphabet = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
    $key = array();
    $alphaLength = strlen($alphabet) - 1;
    for ($i = 0; $i < 20; $i++) {
        $n = rand(0, $alphaLength);
        $key[] = $alphabet[$n];
    }
    return implode($key);
}

// Generate the license key
$license_key = generate_license_key();

// Store the license key in the database (assuming you have a table called 'licenses' with a column called 'key')
// $conn = mysqli_connect("hostname","username","password","database_name");
$query = "INSERT INTO licenses (key) VALUES ('$license_key')";
mysqli_query($conn, $query);

// Display the license key in the frontend
echo "Your license key is: " . $license_key;

?>

Please note that this is just a simple example and not production ready. It is strongly advised to use a library that provides a secure way of generating a license key and also it should be verified on the server side as well when it is being used.

Additionally, it is important to ensure that the code is properly secured against SQL injection and other types of attacks.

It is also important to consider the scalability and maintainability of the code and make sure it meets the requirement and constraints of the use case.