How to use checked() function with multiple check box group? How to properly sanitize that checkbox group?

Question

I'm using the theme options page from the underscores (_s) theme to build a theme options page and I have a group of check boxes on it. In the example below I've just included 2 for simplicity.

My questions are:

1. What goes in the checked() function below to keep each check box checked when saved? I have to check each value against the value in the DB correct?
2. How do I properly sanitize the group of checkboxes? They seem to save to the DB fine, but I know I'm doing something wrong.

Here's each relevant block of code:

// register setting
register_setting(
    'options', // Options group
    'theme_options', // Database option
    'theme_options_validate' // The sanitization callback, see below
);

// add settings field
add_settings_field(
    'post_sharing_networks',
    'Post Sharing Networks To Show',
    'settings_field_post_sharing_networks', // callback below
    'theme_options',
    'general'
);

// set up the array
// more will be added later. 2 shown here for simplicity
function post_sharing_networks() {

    $post_sharing_networks = array(
        'twitter' => array(
            'value' => 'twitter',
            'label' => 'Twitter'
        ),
        'facebook' => array(
            'value' => 'facebook',
            'label' => 'Facebook'
        ),
    );

    return $post_sharing_networks;
}

// callback

function settings_field_post_sharing_networks() {

    $options = get_option( 'theme_options' ); // get array of all the theme options

    $post_sharing_networks = $options['post_sharing_networks']; // this will get array for just the post sharing networks

// eg
    /*
    array (size=2)
  'twitter' => string '1' (length=1)
  'facebook' => string '1' (length=1)
*/

    // builds the checkbox list form the post_sharing_networks() function
    foreach ( post_sharing_networks() as $network ) { ?>    
        <label> 
            <input type="checkbox" value="1" name="theme_options[post_sharing_networks][<?php echo esc_attr( $network['value'] ); ?>]" <?php checked( /* what goes in here for this to work? */ ); ?> />

            <?php echo $network['value']; ?>
        </label>

    <?php }
}


// sanitization callback
function theme_options_validate( $input ) {

    $output = array();

    // how do I properly sanitize the checkbox group?
        // it saves the checkboxes to DB fine currently but I think it can be improved
    $options = get_option( 'theme_options' );

    foreach ( post_sharing_networks() as $network ) {

        if ( isset( $options['post_sharing_networks'] ) )
            $output['post_sharing_networks'] = $input['post_sharing_networks'];

    }

    return apply_filters( 'theme_options_validate', $output, $input );

}   

This is what it looks like when saved into the wp_options table

s:21:"post_sharing_networks";a:2:{s:7:"twitter";s:1:"1";s:8:"facebook";s:1:"1";}

Answer 1

So checked is fairly simple to understand. It compares the first two values. If they're equal, it spits out checked="checked" if they aren't equal nothing happens.

<?php
$saved = 'on';
$compare = 'on'

// spits out checked="checked"
checked($saved, $compare);

$saved = 'off';
// does nothing
checked($saved, $compare);

How you save check boxes it up to you. Because of the way things get serialized, I like to save "checked" values as on and unchecked values as off.

So to sanitize you'd do something like this:

<?php
function wpse74685_clean_option($dirty)
{
    $clean = array(
        'post_sharing_networks'  => array(),
    );

    // $dirty['post_sharing_networks'] will contain the array of
    // of your checkbox values, so check to make sure its there
    // then loop through.
    if(!empty($dirty['post_sharing_networks']))
    {
        // if the box was checked, the value will be there
        // if not nothing will be there.
        foreach(post_sharing_networks() as $network)
            $clean['post_sharing_networks'][$network['value']] = !empty($dirty['post_sharing_networks'][$network['value']]) ? 'on' : 'off';
    }

    return $clean;
}

Then you calls to checked will be something like this:

<input type="checkbox" value="1" 
       name="theme_options[post_sharing_networks][<?php echo esc_attr($network['value']); ?>]"
       <?php checked($post_sharing_networks[$network['value']], 'on'); ?> />