<?php
// Display Fields
add_action('woocommerce_product_options_general_product_data', 'woocommerce_product_custom_fields');

// Save Fields
add_action('woocommerce_process_product_meta', 'woocommerce_product_custom_fields_save');


function woocommerce_product_custom_fields(){
    global $woocommerce, $post;
    echo '<div class="product_custom_field">';
    // Custom Product Checkbox Field
    woocommerce_wp_checkbox(
        array(
            'id'          => '_custom_product_checkbox_field',
            'placeholder' => 'Custom Product Checkbox Field',
            'label'       => __('Custom Product Checkbox Field', 'woocommerce'),
            'desc_tip'    => 'true'
        )
    );
echo '</div>';

}

// Saving Values
function woocommerce_product_custom_fields_save($post_id){
    // Custom Product Text Field
    $woocommerce_custom_product_checkbox_field = $_POST['_custom_product_checkbox_field'];
    if (!empty($woocommerce_custom_product_checkbox_field ))
        update_post_meta($post_id, '_custom_product_checkbox_field', esc_attr($woocommerce_custom_product_checkbox_field ));
}
?>

function add_engrave_text_field() {
    if (is_single('product-url-a')) {
        echo 'Enter your chosen letters:  <span id="character_count"></span>
        <div><table class="variations" cellspacing="0">
            <tbody>
                <tr>
                    <td class="value"><label class="product-custom-text-label" for="custom_text">Custom Text</label></td>
                    <td class="value">
                        <label><input type="text" class="product-counter" name="engrave_text" placeholder="Enter Your Custom Letters ..." maxlength="3" /></label>
                    </td>
                </tr>
            </tbody>
        </table></div>';
    }
}
add_action( 'woocommerce_before_add_to_cart_button', 'add_engrave_text_field', 0 );

// Display Fields
add_action('woocommerce_product_options_general_product_data', 'product_custom_fields_add');
function product_custom_fields_add(){

    echo '<div class="product_custom_field">';

    // Custom Product Checkbox Field
    woocommerce_wp_checkbox(
        array(
            'id'        => '_engrave_text_option',
            'desc'      =>  __('set custom Engrave text field', 'woocommerce'),
            'label'     => __('Display custom Engrave text field', 'woocommerce'),
            'desc_tip'  => 'true'
        )
    );

    echo '</div>';
}

// Save Fields
add_action('woocommerce_process_product_meta', 'product_custom_fields_save');
function product_custom_fields_save($post_id){
    // Custom Product Text Field
    if ( ! empty( $_POST['_engrave_text_option'] ) )
        update_post_meta($post_id, '_engrave_text_option', esc_attr( $_POST['_engrave_text_option'] ));
}

add_action( 'woocommerce_before_add_to_cart_button', 'add_engrave_text_field', 0 );
function add_engrave_text_field() {
    global $post;

    // Get the checkbox value
    $engrave_option = get_post_meta( $post->ID, '_engrave_text_option', true );

    // If is single product page and have the "engrave text option" enabled we display the field
    if ( is_product() && ! empty($engrave_option) ) {

        ?>
        <div>
            <label class="product-custom-text-label" for="engrave_text"><?php _e( 'Engraving option:', 'woocommerce'); ?><br>
                <input style="min-width:220px" type="text" class="product-counter" name="engrave_text" placeholder="<?php _e( 'Enter Your Custom Letters ...', 'woocommerce'); ?>" maxlength="3" />
            </label>
        </div><br>
        <?php
    }
}

add_action( 'woocommerce_before_add_to_cart_button', 'add_engrave_text_field', 0 );
function add_engrave_text_field() {
    global $post;

    // Get the checkbox value
    $engrave_option = get_post_meta( $post->ID, '_engrave_text_option', true );

    // If is single product page and have the "engrave text option" enabled we display the field
    if ( is_product() && ! empty($engrave_option) ) {

        ?>
        <div>
            <label class="product-custom-text-label" for="engrave_text"><?php _e( 'Engraving option:', 'woocommerce'); ?><br>
                <input style="min-width:220px" type="text" class="product-counter" name="engrave_text" placeholder="<?php _e( 'Enter Your Custom Letters ...', 'woocommerce'); ?>" maxlength="3" />
            </label>
        </div><br>
        <?php
    }
}