How can PHP developers protect their software from being illegally distributed or used on multiple websites?

To protect PHP software from being illegally distributed or used on multiple websites, developers can implement license key verification. This involves generating a unique license key for each installation and validating it against a centralized server to ensure it is legitimate. By checking the license key on each request, developers can prevent unauthorized use of their software.

// Function to validate license key
function validate_license_key($license_key) {
    $valid_keys = array('abc123', 'def456', 'ghi789'); // Array of valid license keys
    
    if (in_array($license_key, $valid_keys)) {
        return true; // Valid license key
    } else {
        return false; // Invalid license key
    }
}

// Example of validating license key
$license_key = $_POST['license_key']; // Get license key from request
if (validate_license_key($license_key)) {
    // Software is licensed, continue execution
} else {
    // Invalid license key, display error message or restrict access
}