Temporary Fluent Forms workaround to preserve plus-addressed email values in redirect query strings.

fluentforms-preserve-plus-email-redirect.php

<?php

// Temporary Fluent Forms workaround:
// Preserve plus-addressed emails when passing email through redirect query string.
add_filter('fluentform/submission_confirmation', function ($returnData, $form, $confirmation, $insertId = null, $formData = []) {
    $sourceFormId = 29;        // Form #1 ID
    $emailFieldName = 'email'; // Form #1 email field name
    $queryKey = 'email';       // Redirect URL query key: ?email=

    if ((int) $form->id !== $sourceFormId || empty($returnData['redirectUrl'])) {
        return $returnData;
    }

    if (!is_array($formData) || empty($formData[$emailFieldName])) {
        return $returnData;
    }

    $email = sanitize_email($formData[$emailFieldName]);

    if (!$email || !is_email($email)) {
        return $returnData;
    }

    $returnData['redirectUrl'] = wpmn_fix_plus_email_redirect_url(
        $returnData['redirectUrl'],
        $queryKey,
        $email
    );

    return $returnData;
}, 10, 5);

if (!function_exists('wpmn_fix_plus_email_redirect_url')) {
    function wpmn_fix_plus_email_redirect_url($url, $queryKey, $value)
    {
        $fragment = '';

        if (strpos($url, '#') !== false) {
            list($url, $fragment) = explode('#', $url, 2);
            $fragment = '#' . $fragment;
        }

        $encodedKey = rawurlencode($queryKey);
        $encodedValue = rawurlencode($value);
        $pattern = '/([?&])' . preg_quote($queryKey, '/') . '=[^&#]*/';

        if (preg_match($pattern, $url)) {
            $url = preg_replace_callback($pattern, function ($matches) use ($encodedKey, $encodedValue) {
                return $matches[1] . $encodedKey . '=' . $encodedValue;
            }, $url, 1);
        } else {
            $separator = strpos($url, '?') === false ? '?' : '&';
            $url .= $separator . $encodedKey . '=' . $encodedValue;
        }

        return $url . $fragment;
    }
}