source: trunk/www.guidonia.net/wp/wp-content/plugins/contact-form-7/modules/checkbox.php@ 44

<?php
/**
** A base module for [checkbox], [checkbox*], and [radio]
**/

/* Shortcode handler */

function wpcf7_checkbox_shortcode_handler( $tag ) {
        global $wpcf7_contact_form;

        if ( ! is_array( $tag ) )
                return '';

        $type = $tag['type'];
        $name = $tag['name'];
        $options = (array) $tag['options'];
        $values = (array) $tag['values'];

        if ( empty( $name ) )
                return '';

        $atts = '';
        $id_att = '';
        $class_att = '';

        $defaults = array();

        $label_first = false;
        $use_label_element = false;

        if ( 'checkbox*' == $type )
                $class_att .= ' wpcf7-validates-as-required';

        if ( 'checkbox' == $type || 'checkbox*' == $type )
                $class_att .= ' wpcf7-checkbox';

        if ( 'radio' == $type )
                $class_att .= ' wpcf7-radio';

        foreach ( $options as $option ) {
                if ( preg_match( '%^id:([-0-9a-zA-Z_]+)$%', $option, $matches ) ) {
                        $id_att = $matches[1];

                } elseif ( preg_match( '%^class:([-0-9a-zA-Z_]+)$%', $option, $matches ) ) {
                        $class_att .= ' ' . $matches[1];

                } elseif ( preg_match( '/^default:([0-9_]+)$/', $option, $matches ) ) {
                        $defaults = explode( '_', $matches[1] );

                } elseif ( preg_match( '%^label[_-]?first$%', $option ) ) {
                        $label_first = true;

                } elseif ( preg_match( '%^use[_-]?label[_-]?element$%', $option ) ) {
                        $use_label_element = true;

                }
        }

        if ( $id_att )
                $atts .= ' id="' . trim( $id_att ) . '"';

        if ( $class_att )
                $atts .= ' class="' . trim( $class_att ) . '"';

        $multiple = preg_match( '/^checkbox[*]?$/', $type ) && ! preg_grep( '%^exclusive$%', $options );

        $html = '';

        if ( preg_match( '/^checkbox[*]?$/', $type ) && ! $multiple && WPCF7_LOAD_JS )
                $onclick = ' onclick="wpcf7ExclusiveCheckbox(this);"';

        $input_type = rtrim( $type, '*' );

        $posted = is_a( $wpcf7_contact_form, 'WPCF7_ContactForm' ) && $wpcf7_contact_form->is_posted();

        foreach ( $values as $key => $value ) {
                $checked = false;

                if ( in_array( $key + 1, (array) $defaults ) )
                        $checked = true;

                if ( $posted) {
                        if ( $multiple && in_array( esc_sql( $value ), (array) $_POST[$name] ) )
                                $checked = true;
                        if ( ! $multiple && $_POST[$name] == esc_sql( $value ) )
                                $checked = true;
                }

                $checked = $checked ? ' checked="checked"' : '';

                if ( is_array( $tag['labels'] ) && isset( $tag['labels'][$key] ) )
                        $label = $tag['labels'][$key];
                else
                        $label = $value;

                if ( $label_first ) { // put label first, input last
                        $item = '<span class="wpcf7-list-item-label">' . esc_html( $label ) . '</span>&nbsp;';
                        $item .= '<input type="' . $input_type . '" name="' . $name . ( $multiple ? '[]' : '' ) . '" value="' . esc_attr( $value ) . '"' . $checked . $onclick . ' />';
                } else {
                        $item = '<input type="' . $input_type . '" name="' . $name . ( $multiple ? '[]' : '' ) . '" value="' . esc_attr( $value ) . '"' . $checked . $onclick . ' />';
                        $item .= '&nbsp;<span class="wpcf7-list-item-label">' . esc_html( $label ) . '</span>';
                }

                if ( $use_label_element )
                        $item = '<label>' . $item . '</label>';

                $item = '<span class="wpcf7-list-item">' . $item . '</span>';
                $html .= $item;
        }

        $html = '<span' . $atts . '>' . $html . '</span>';

        $validation_error = '';
        if ( is_a( $wpcf7_contact_form, 'WPCF7_ContactForm' ) )
                $validation_error = $wpcf7_contact_form->validation_error( $name );

        $html = '<span class="wpcf7-form-control-wrap ' . $name . '">' . $html . $validation_error . '</span>';

        return $html;
}

wpcf7_add_shortcode( 'checkbox', 'wpcf7_checkbox_shortcode_handler', true );
wpcf7_add_shortcode( 'checkbox*', 'wpcf7_checkbox_shortcode_handler', true );
wpcf7_add_shortcode( 'radio', 'wpcf7_checkbox_shortcode_handler', true );


/* Validation filter */

function wpcf7_checkbox_validation_filter( $result, $tag ) {
        global $wpcf7_contact_form;

        $type = $tag['type'];
        $name = $tag['name'];
        $values = $tag['values'];

        if ( is_array( $_POST[$name] ) ) {
                foreach ( $_POST[$name] as $key => $value ) {
                        $value = stripslashes( $value );
                        if ( ! in_array( $value, (array) $values ) ) // Not in given choices.
                                unset( $_POST[$name][$key] );
                }
        } else {
                $value = stripslashes( $_POST[$name] );
                if ( ! in_array( $value, (array) $values ) ) //  Not in given choices.
                        $_POST[$name] = '';
        }

        if ( 'checkbox*' == $type ) {
                if ( empty( $_POST[$name] ) ) {
                        $result['valid'] = false;
                        $result['reason'][$name] = $wpcf7_contact_form->message( 'invalid_required' );
                }
        }

        return $result;
}

add_filter( 'wpcf7_validate_checkbox', 'wpcf7_checkbox_validation_filter', 10, 2 );
add_filter( 'wpcf7_validate_checkbox*', 'wpcf7_checkbox_validation_filter', 10, 2 );
add_filter( 'wpcf7_validate_radio', 'wpcf7_checkbox_validation_filter', 10, 2 );

?>