populate-givewp-dropdown-from-url.php

<?php
// URLs make values lower case, so values in the select field must be lowercase
// CSS can somewhat help those values 

add_action('givewp_donation_form_schema', function($form) {
    
    // 1. Define the variable here
    $target_param = 'test_dropdown'; 

    // Get field by name attribute using the variable
    $field = $form->getNodeByName($target_param);

    // if the field is not found, return
    if (!$field) {
        return;
    }

    // get the current default value
    $defaultValue = $field->getDefaultValue();

    // if the value is set in the query string, use that as the default
    // eg. yoursite.com/?value=somevalue
    if (isset($_GET[$target_param])) {
        $defaultValue = give_clean($_GET[$target_param]);
    
    // if the value is set in the referrer query string, the form is being embed so we should use that instead
    } elseif (isset($_SERVER['HTTP_REFERER'])) {
        parse_str(parse_url($_SERVER['HTTP_REFERER'], PHP_URL_QUERY), $query);
        
        // Added isset() here to prevent "Undefined index" warnings
        if (isset($query[$target_param])) {
            $defaultValue = give_clean($query[$target_param]);
        }
    }

    // we don't want to set the default value if it's the same as the field's default value
    if ($defaultValue === $field->getDefaultValue()){
        return;
    }

    $field->defaultValue($defaultValue);
});