Flexible Checkout Fields - custom validation method

functions.php

<?php // do not copy this line if already opened <?php tag

function example_function_is_integer( $field_label, $value ) {
    if ( ! ( (is_numeric($value) ? intval($value) == $value : false) ) ) {
        wc_add_notice( sprintf( '%s is not integer!', '<strong>' . $field_label . '</strong>' ), 'error' );
    }
}

add_filter( 'flexible_checkout_fields_custom_validation', 'example_flexible_checkout_fields_custom_validation' );
function example_flexible_checkout_fields_custom_validation( $custom_validation ) {
    $custom_validation['is_integer'] = array(
        'label'     => 'Is integer',
        'callback'  => 'example_function_is_integer'
    );
    return $custom_validation;
}