请尝试以下代码。

add_action( \'add_meta_boxes\', function() {
    add_meta_box( \'custom-metabox\', \'Select values\', \'fill_metabox\', \'post\', \'normal\' );
});

function fill_metabox( $post ) {
    wp_nonce_field( basename(__FILE__), \'mam_nonce\' );

    // How to use \'get_post_meta()\' for multiple checkboxes as array?
    $postmeta = maybe_unserialize( get_post_meta( $post->ID, \'elements\', true ) );

    // Our associative array here. id = value
    $elements = array(
        \'apple\'  => \'Apple\',
        \'orange\' => \'Orange\',
        \'banana\' => \'Banana\'
    );

    // Loop through array and make a checkbox for each element
    foreach ( $elements as $id => $element) {

        // If the postmeta for checkboxes exist and 
        // this element is part of saved meta check it.
        if ( is_array( $postmeta ) && in_array( $id, $postmeta ) ) {
            $checked = \'checked="checked"\';
        } else {
            $checked = null;
        }
        ?>

        <p>
            <input  type="checkbox" name="multval[]" value="<?php echo $id;?>" <?php echo $checked; ?> />
            <?php echo $element;?>
        </p>

        <?php
    }
}


add_action( \'save_post\', function( $post_id ) {
    $is_autosave = wp_is_post_autosave( $post_id );
    $is_revision = wp_is_post_revision( $post_id );
    $is_valid_nonce = ( isset( $_POST[ \'mam_nonce\' ] ) && wp_verify_nonce( $_POST[ \'mam_nonce\' ], basename( __FILE__ ) ) ) ? \'true\' : \'false\';

    if ( $is_autosave || $is_revision || !$is_valid_nonce ) {
        return;
    }

    // If the checkbox was not empty, save it as array in post meta
    if ( ! empty( $_POST[\'multval\'] ) ) {
        update_post_meta( $post_id, \'elements\', $_POST[\'multval\'] );

    // Otherwise just delete it if its blank value.
    } else {
        delete_post_meta( $post_id, \'elements\' );
    }

});