Frontend Form Checkbox Update ACF Field

To be compatible with ACF you should first find out how ACF is saving checkboxes values to database. As I now this is serialized array. When we know that we must save our frontend checkboxes in the same way. The good news is you can pass only array to update_post_meta and WordPress will take care of serialization.

The problem with your code is that you probably not passing array of checkbox values. You should investigate this part: esc_attr(strip_tags($_POST['customMetaAppliances[]'])).

Two things which bother me here:

• esc_attr and strip_tags functions are expecting string as argument not an array they may return unexpected results
• if you want to get array of checkboxes from post request you should use field name without square brackets customMetaAppliances

I think you can replace saving part with code:

$customMetaAppliances = filter_input( INPUT_POST, 'customMetaAppliances', FILTER_SANITIZE_STRING, FILTER_REQUIRE_ARRAY );
update_field( 'customMetaAppliances', $customMetaAppliances, $post_id );

Personaly when I have installed ACF I use update_field instead of update_post_meta.

I created fully working class which will display form on the frontend after post content and save values to post ACF fields on submit.

class WPSE_287946_Form {

    /**
     * Class constructor
     */
    public function __construct() {

        $this->define_hooks();
    }

    public function save_form() {

        if( isset( $_POST['save'] ) ) { // Submit button

            $post_id     = filter_input( INPUT_POST, 'id', FILTER_SANITIZE_NUMBER_INT );
            $name        = filter_input( INPUT_POST, 'name', FILTER_SANITIZE_STRING );
            $color       = filter_input( INPUT_POST, 'color', FILTER_SANITIZE_STRING );
            $accessories = filter_input( INPUT_POST, 'accessories', FILTER_SANITIZE_STRING, FILTER_REQUIRE_ARRAY );

            update_field( 'name', $name, $post_id );
            update_field( 'color', $color, $post_id );
            update_field( 'accessories', $accessories, $post_id );

            // Redirect user because POST request on single post page will trigger 404 page.
            wp_redirect( get_permalink( $post_id ) );
        }
    }

    /**
     * Display form
     */
    public function display_form( $content ) {

        $name        = get_field( 'name' );
        $color       = get_field( 'color' );
        $accessories = get_field( 'accessories' );

        ob_start();

        ?>

        <form method="post">
            <p>
                <?php $this->display_text( 'name', 'Name', $name ); ?>
            </p>
            <p>
                <?php $this->display_select( 'color', 'Color', $this->get_available_colors(), $color ); ?>
            </p>
            <p>
                <?php $this->display_checkboxes( 'accessories', 'Accessories', $this->get_available_accessories(), $accessories ); ?>
            </p>
            <p>
                <input type="hidden" name="id" value="<?php esc_attr_e( get_the_ID() ) ?>">
                <input type="submit" name="save" value="Submit" />
            </p>
        </form>

        <?php

        $output = ob_get_contents();
        ob_end_clean();

        return $content . $output;
    }

    /**
     * Display text field
     */
    private function display_text( $name, $label, $value="" ) {
        ?>
        <label><?php esc_html_e( $label, 'wpse_287946' ) ?></label>
        <input type="text" name="<?php esc_attr_e( $name ) ?>" value="<?php esc_attr_e( $value ); ?>">
        <?php
    }

    /**
     * Display select field
     */
    private function display_select( $name, $label, $options, $value="" ) {
        ?>
        <label><?php esc_html_e( $label, 'wpse_287946' ) ?></label>
        <select name="<?php esc_attr_e( $name ) ?>">
            <?php $this->display_options( $options, $value ); ?>
        </select>
        <?php
    }

    /**
     * Display options
     */
    private function display_options( $options, $value ) {

        foreach( $options as $option_value => $option_label ):
            $selected = ( $option_value === $value ) ? ' selected' : '';
            ?>
            <option value="<?php esc_attr_e( $option_value ) ?>"<?php esc_attr_e( $selected ) ?>><?php esc_html_e( $option_label, 'wpse_287946' ) ?></option>
            <?php

        endforeach;
    }

    /**
     * Display checkboxes field
     */
    private function display_checkboxes( $name, $label, $options, $values = array() ) {

        $name .= '[]';

        ?>
        <label><?php esc_html_e( $label, 'wpse_287946' ) ?></label>
        <?php

        foreach ( $options as $option_value => $option_label ):
            $this->display_checkbox( $name, $option_label, $option_value, $values );
        endforeach;
    }

    /**
     * Display single checkbox field
     */
    private function display_checkbox( $name, $label, $available_value, $values = array() ) {
        $checked = ( in_array($available_value, $values) ) ? ' checked' : '';
        ?>
        <label>
            <input type="checkbox" name="<?php esc_attr_e( $name ) ?>" value="<?php esc_attr_e( $available_value ) ?>"<?php esc_attr_e( $checked ) ?>>
            <?php esc_html_e( $label, 'wpse_287946' ) ?>
        </label>
        <?php
    }

    /**
     * Get available colors
     */
    private function get_available_colors() {

        return array(
            'red' => 'Red',
            'blue' => 'Blue',
            'green' => 'Green',
        );
    }

    /**
     * Get available accessories
     */
    private function get_available_accessories() {

        return array(
            'case' => 'Case',
            'tempered_glass' => 'Tempered glass',
            'headphones' => 'Headphones',
        );
    }

    /**
     * Define hooks related to plugin
     */
    private function define_hooks() {

        /**
         * Add action to save form
         */
        add_action( 'wp', array( $this, 'save_form' ) );

        /**
         * Add filter to display form
         */
        add_filter( 'the_content', array( $this, 'display_form' ) );
    }
}

new WPSE_287946_Form();