[Steven Zahm]

@ James

Adding a checkbox is only mildly more difficult. Here’s the code that to add two text fields and three checkboxes:

$atts = array(
    'title'    => 'Metabox Name',         // Change this to a name which applies to your project.
    'id'       => 'custom_one',           // Change this so it is unique to you project.
    'context'  => 'normal',
    'priority' => 'core',
    'fields'   => array(
        array(
            'name'       => 'Field Name One', // Change this field name to something which applies to you project.
            'show_label' => TRUE,             // Whether or not to display the 'name'. Changing it to false will suppress the name.
            'id'         => 'field_id_1',     // Change this so it is unique to you project. Each field id MUST be unique.
            'type'       => 'text',           // This is the field type being added.
            'size'       => 'regular',        // This can be changed to one of the following: 'small', 'regular', 'large'
        ),
        array(
            'name'       => 'Field Name Two', // Change this field name to something which applies to you project.
            'show_label' => TRUE,             // Whether or not to display the 'name'. Changing it to false will suppress the name.
            'id'         => 'field_id_2',     // Change this so it is unique to you project. Each field id MUST be unique.
            'type'       => 'text',           // This is the field type being added.
            'size'       => 'regular',        // This can be changed to one of the following: 'small', 'regular', 'large'
        ),
        array(
            'name'       => 'Field Name Three', // Change this field name to something which applies to you project.
            'show_label' => TRUE,               // Whether or not to display the 'name'. Changing it to false will suppress the name.
            'desc'       => 'Checkbox Description',
            'id'         => 'field_id_3',       // Change this so it is unique to you project. Each field id MUST be unique.
            'type'       => 'checkbox',         // This is the field type being added.
        ),
        array(
            'name'       => 'Field Name Four',  // Change this field name to something which applies to you project.
            'show_label' => TRUE,               // Whether or not to display the 'name'. Changing it to false will suppress the name.
            'desc'       => 'Checkbox Description',
            'id'         => 'field_id_4',       // Change this so it is unique to you project. Each field id MUST be unique.
            'type'       => 'checkbox',         // This is the field type being added.
        ),
        array(
            'name'       => 'Field Name Five',  // Change this field name to something which applies to you project.
            'show_label' => TRUE,               // Whether or not to display the 'name'. Changing it to false will suppress the name.
            'desc'       => 'Checkbox Description',
            'id'         => 'field_id_5',       // Change this so it is unique to you project. Each field id MUST be unique.
            'type'       => 'checkbox',         // This is the field type being added.
        ),
    ),
);

Displaying this in the template is a bit different because the checkbox is saved as either 1 (checked) or 0 (not checked). So you would want to do this to display it in the template:

$checked = $entry->getMeta(
        array(
            'key'    => 'field_id', // This should match exactly the field id used when registering the custom field.
            'single' => TRUE        // Do not change this.
        )
    );

echo if ( $checked ) '<p>Checkbox was checked.</p>';

Hope that helps!