if (!function_exists('get_previous_post_link')) {
    function get_previous_post_link($label1, $label2, $label3)
    {
        ob_start();
        previous_post_link($label1, $label2, $label3);
        return ob_get_clean();
    }
}

if (!function_exists('get_next_post_link')) {
    function get_next_post_link($label1, $label2, $label3)
    {
        ob_start();
        next_post_link($label1, $label2, $label3);
        return ob_get_clean();
    }
}