WooCommerce Add Custom Fields to Products

functions.php

<?php

// For variations
add_action( 'woocommerce_variation_options_pricing', 'yanco_add_custom_field_to_variations', 10, 3 );
function yanco_add_custom_field_to_variations( $loop, $variation_data, $variation ) {
    woocommerce_wp_text_input( array(
            'id' => 'custom_field[' . $loop . ']',
            'class' => 'short',
            'label' => __( 'Custom Field', 'woocommerce' ),
            'value' => get_post_meta( $variation->ID, 'custom_field', true )
        )
    );
}

add_action( 'woocommerce_save_product_variation', 'yanco_save_custom_field_variations', 10, 2 );
function yanco_save_custom_field_variations( $variation_id, $i ) {
    $custom_field = $_POST['custom_field'][$i];
    if ( isset( $custom_field ) ) {
        update_post_meta( $variation_id, 'custom_field', esc_attr( $custom_field ) );
    }
}

add_filter( 'woocommerce_available_variation', 'yanco_add_custom_field_variation_data' );
function yanco_add_custom_field_variation_data( $variations ) {
    $variations['custom_field'] = '<div class="woocommerce_custom_field">Custom Field: <span>' . get_post_meta( $variations[ 'variation_id' ], 'custom_field', true ) . '</span></div>';
    return $variations;
}

// For Singular Product
add_action( 'woocommerce_product_options_pricing', 'yanco_add_custom_field_to_products' );
function yanco_add_custom_field_to_products() {          
    woocommerce_wp_text_input( array( 
        'id' => 'custom-field-id', 
        'class' => 'short wc_input_price',
        'label' => __( 'Custom Field', 'woocommerce' ) . ' (' . get_woocommerce_currency_symbol() . ')',
        'data_type' => 'price', 
    ));      
}

add_action( 'save_post_product', 'yanco_save_custom_field' );
function yanco_save_custom_field( $product_id ) {
    global $pagenow, $typenow;

    if ( 'post.php' !== $pagenow || 'product' !== $typenow ) {
        return;
    } 
    
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
        return;
    } 
    
    if ( isset( $_POST['custom-field-id'] ) ) {
        if ( $_POST['custom-field-id'] ) {
            update_post_meta( $product_id, 'custom-field-meta-key', $_POST['custom-field-id'] );
        }
        
    } else {
        delete_post_meta( $product_id, 'custom-field-meta-key' );
    }
}

add_action( 'woocommerce_single_product_summary', 'yanco_display_custom_field', 9 );
function yanco_display_custom_field() {
    global $product;

    if ( $product->get_type() <> 'variable' && $custom_field_value = get_post_meta( $product->get_id(), 'custom-field-meta-key', true ) ) {
        echo '<div class="woocommerce_custom_field">';
        _e( 'Custom Field: ', 'woocommerce' );
        echo '<span>' . wc_price( $custom_field_value ) . '</span>';
        echo '</div>';
    }
}