Metabox

class google_adwords_options {

	public function __construct() {

		if ( is_admin() ) {
			add_action( 'load-post.php',     array( $this, 'init_metabox' ) );
			add_action( 'load-post-new.php', array( $this, 'init_metabox' ) );
		}

	}

	public function init_metabox() {

		add_action( 'add_meta_boxes',        array( $this, 'add_metabox' )         );
		add_action( 'save_post',             array( $this, 'save_metabox' ), 10, 2 );

	}

	public function add_metabox() {

		add_meta_box(
			'google_adwords_options',
			__( 'Google Adwords Options', 'google_adwords_prev' ),
			array( $this, 'render_metabox' ),
			'post',
			'advanced',
			'default'
		);

	}

	public function render_metabox( $post ) {

		// Add nonce for security and authentication.
		wp_nonce_field( 'custom_nonce_action', 'custom_nonce' );

		// Retrieve an existing value from the database.
		$custom_language = get_post_meta( $post->ID, 'custom_language', true );
		$custom_devices = get_post_meta( $post->ID, 'custom_devices', true );

		// Set default values.
		if( empty( $custom_language ) ) $custom_language = '';
		if( empty( $custom_devices ) ) $custom_devices = array();

		// Form fields.
		echo '<table class="form-table">';

		echo '	<tr>';
		echo '		<th><label for="custom_language" class="custom_language_label">' . __( 'Sprache', 'google_adwords_prev' ) . '</label></th>';
		echo '		<td>';
		echo '			<label><input type="radio" name="custom_language" class="custom_language_field" value="custom_german" ' . checked( $custom_language, 'custom_german', false ) . '> ' . __( 'Deutsch', 'google_adwords_prev' ) . '</label><br>';
		echo '			<label><input type="radio" name="custom_language" class="custom_language_field" value="custom_english" ' . checked( $custom_language, 'custom_english', false ) . '> ' . __( 'Englisch', 'google_adwords_prev' ) . '</label><br>';
		echo '			<p class="description">' . __( 'Wählen Sie eine Sprache', 'google_adwords_prev' ) . '</p>';
		echo '		</td>';
		echo '	</tr>';

		echo '	<tr>';
		echo '		<th><label for="custom_devices" class="custom_devices_label">' . __( 'Geräte', 'google_adwords_prev' ) . '</label></th>';
		echo '		<td>';
		echo '			<label><input type="checkbox" name="custom_devices[]" class="custom_devices_field" value="' . esc_attr( 'mobile' ) . '" ' . ( in_array( 'mobile', $custom_devices )? 'checked="checked"' : '' ) . '> ' . __( 'Mobile Device', 'google_adwords_prev' ) . '</label><br>';
		echo '			<label><input type="checkbox" name="custom_devices[]" class="custom_devices_field" value="' . esc_attr( 'desktop' ) . '" ' . ( in_array( 'desktop', $custom_devices )? 'checked="checked"' : '' ) . '> ' . __( 'Desktop Device', 'google_adwords_prev' ) . '</label><br>';
		echo '			<p class="description">' . __( 'Wählen Sie die Geräte', 'google_adwords_prev' ) . '</p>';
		echo '		</td>';
		echo '	</tr>';

		echo '</table>';

	}

	public function save_metabox( $post_id, $post ) {

		// Add nonce for security and authentication.
		$nonce_name   = isset( $_POST['custom_nonce'] ) ? $_POST['custom_nonce'] : '';
		$nonce_action = 'custom_nonce_action';

		// Check if a nonce is set.
		if ( ! isset( $nonce_name ) )
			return;

		// Check if a nonce is valid.
		if ( ! wp_verify_nonce( $nonce_name, $nonce_action ) )
			return;

		// Sanitize user input.
		$custom_new_language = isset( $_POST[ 'custom_language' ] ) ? $_POST[ 'custom_language' ] : '';
		$custom_new_devices = isset( $_POST[ 'custom_devices' ] ) ? array_intersect( (array) $_POST[ 'custom_devices' ], array( 'mobile','desktop' ) )  : array();

		// Update the meta field in the database.
		update_post_meta( $post_id, 'custom_language', $custom_new_language );
		update_post_meta( $post_id, 'custom_devices', $custom_new_devices );

	}

}

new google_adwords_options;