Checked() showing up twice in custom fields

Question

I've successfully applied a meta box and checkbox field that can save my settings. However, there is a bit of leftover code that seems to show up on my meta box field.

The checked='checked' shows up correctly in the input checkbox, but it also appears again a standalone preceding the input tag, which I don't want. This is the output:

checked='checked'<input type="checkbox" id="my_meta_box_check1" name="my_meta_box_check1"  checked='checked' /><label for="my_meta_box_check1">Do not check this</label>

In my functions.php, I have used the echo output method along with concatenation:

/* Meta Box: Create container */
add_action( 'add_meta_boxes', 'my_meta_box_add' );
function my_meta_box_add() {
    add_meta_box( 'my-meta-box', 'Rockwood Options', 'my_meta_box_fields', 'page', 'side', 'high' );
}

/* Meta Box: Create individual fields */
function my_meta_box_fields($post) {
    $values = get_post_custom( $post->ID );
    $check = isset( $values['my_meta_box_check1'] ) ? esc_attr( $values['my_meta_box_check1'][0] ) : '';
    wp_nonce_field( 'my_meta_box_nonce', 'meta_box_nonce' );
    echo '<input type="checkbox" id="my_meta_box_check1" name="my_meta_box_check1" ' . checked( $check, "on" ) . ' />';
    echo '<label for="my_meta_box_check1">Do not check this</label>';
}

/* Meta Box: Save fields */
add_action( 'save_post', 'my_meta_box_save' );
function my_meta_box_save( $post_id ) {
    // Bail if we're doing an auto save
    if( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) return;
    // if our nonce isn't there, or we can't verify it, bail
    if( !isset( $_POST['meta_box_nonce'] ) || !wp_verify_nonce( $_POST['meta_box_nonce'], 'my_meta_box_nonce' ) ) return;
    // if our current user can't edit this post, bail
    if( !current_user_can( 'edit_post', $post_id ) ) return;
    // This is purely my personal preference for saving check-boxes
    $chk = isset( $_POST['my_meta_box_check1'] ) && $_POST['my_meta_box_check1'] ? 'on' : 'off';
        update_post_meta( $post_id, 'my_meta_box_check1', $chk );
}

I suspect I'm applying the concatenation incorrectly. Anyone have any tips on this?

Answer 1

checked() will echo the result by default. That's the problem.

To concat, you need to disable echo functionality by specify third param to false:

echo '<input type="checkbox" id="my_meta_box_check1" name="my_meta_box_check1" ' . checked( $check, "on", false ) . '/>';