Validate that value exists in the entries for another form

quform-custom-code.php

add_filter('quform_element_valid_2_4', function ($valid, $value, Quform_Element_Field $element) {
    if ($element->getForm()->config('environment') != 'frontend') {
        return $valid;
    }

    global $wpdb;
    $repository = quform('repository');
    $emailElementId = '1_3';
    list($formId, $elementId) = explode('_', $emailElementId);

    $query = "SELECT `e`.`id` FROM `" . $repository->getEntryDataTableName() . "` ed LEFT JOIN `" .
        $repository->getEntriesTableName() . "` e ON `ed`.`entry_id` = `e`.`id`
WHERE `e`.`form_id` = %d
AND `ed`.`element_id` = %d
AND `ed`.`value` = %s";

    $args = array(
        $formId,
        $elementId,
        $value
    );

    $result = $wpdb->get_row($wpdb->prepare($query, $args));

    if ($result == null) {
        $element->addError('Email address not found');
        $valid = false;
    }

    return $valid;
}, 10, 3);

The code can be added to the WP theme functions.php file or using a plugin like My Custom Functions.

• On line 1 replace 2_4 with the unique ID of the field you want to validate
• On line 8 replace 1_3 with the unique ID of the field to search the entries for
• On line 26 change the validation error message if necessary