<?php
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?

    $elements = get_elements(); //returns an associative array
    foreach ( $elements as $element) {
        ?>

        <p>
            <input
                type  = "checkbox"
                name  = "multval[]"
                value = <?php echo $element;?>
                //How to use checked() function in case of
                //multiple checkbox as arrays
            >
            <?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;
    }

    /*How to run a loop to save values for each checkbox?*/
});

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' );
    }

});