save checkbox meta box and use it in wordpress - php

Question

I made a checkbox meta box in my functions.php file but I have two problems. I can't save if checkbox is checked or not for the next times. and I want to show something in div if it was checked.
<?php
function filmview_ext_info_meta_box() {
add_meta_box(
'filmview_ext_info_meta_box',
__( 'Extra info', 'filmview_ext_info_meta' ),
'filmview_ext_info_meta_html',
'filmview',
'normal',
'high'
);
}
add_action( 'add_meta_boxes', 'filmview_ext_info_meta_box' );
function filmview_ext_info_meta_html( $post) {
wp_nonce_field( '_filmview_ext_info_meta_nonce', 'filmview_ext_info_meta_nonce' ); ?>
<p>
<input type="checkbox" id="filmview_ext_info_meta_sub" name="filmview_ext_info_meta_sub" <?php checked( $check, 'on' ); ?> />
<label for="filmview_ext_info_meta_sub">Does it have subtitle?</label>
</p>
<?php
}
function filmview_info_meta_save( $post_id ) {
if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) return;
if ( ! isset( $_POST['filmview_info_meta_nonce'] ) || ! wp_verify_nonce( $_POST['filmview_info_meta_nonce'], '_filmview_info_meta_nonce' ) ) return;
if ( ! current_user_can( 'edit_post', $post_id ) ) return;
$chk = isset( $_POST['filmview_ext_info_meta_sub'] ) ? 'on' : 'off';
update_post_meta( $post_id, 'filmview_ext_info_meta_sub', $chk );
}
add_action( 'save_post', 'filmview_info_meta_save' );
?>

Answer 1

What I can see is that:
1) You init $chk in the function: filmview_info_meta_save
2) You are using (with your provided code, an undefined variable ) $check
Try to read it from the database instead of of $_POST which is only present in the page after you submit the form, this should Do:
function filmview_ext_info_meta_html( $post) {
wp_nonce_field( '_filmview_ext_info_meta_nonce', 'filmview_ext_info_meta_nonce' );
$chk = get_post_meta($post->ID, 'filmview_ext_info_meta_sub', true);
?>
<p>
<input type="checkbox" id="filmview_ext_info_meta_sub" name="filmview_ext_info_meta_sub" <?php checked( $chk[0], 'on' ); ?> />
<label for="filmview_ext_info_meta_sub">Does it have subtitle?</label>
</p>
<?php
}