save_woocommerce_variations.php

<?php
// Admin side Variable product fields

add_action( 'woocommerce_product_after_variable_attributes', 'ci_variation_settings_fields', 10, 3 );
// Save Variation Settings
add_action( 'woocommerce_save_product_variation', 'ci_save_variation_settings_fields', 10, 2 );

// Create new fields for variations
function ci_variation_settings_fields( $loop, $variation_data, $variation ) {
  // Text Field
  woocommerce_wp_text_input( 
    array( 
      'id'          => 'ci_text_field[' . $variation->ID . ']', 
      'label'       => __( 'My Text Field', 'woocommerce' ), 
      'placeholder' => 'Enter text field value',
      'desc_tip'    => 'true',
      'description' => __( 'Enter the text field value here.', 'woocommerce' ),
      'value'       => get_post_meta( $variation->ID, 'ci_text_field', true )
    )
  );
  // Number Field
  woocommerce_wp_text_input( 
    array( 
      'id'          => 'ci_number_field[' . $variation->ID . ']', 
      'label'       => __( 'My Number Field', 'woocommerce' ), 
      'desc_tip'    => 'true',
      'description' => __( 'Enter the custom number here.', 'woocommerce' ),
      'value'       => get_post_meta( $variation->ID, 'ci_number_field', true ),
      'custom_attributes' => array(
              'step'  => 'any',
              'min' => '0'
            ) 
    )
  );
  // Textarea
  woocommerce_wp_textarea_input( 
    array( 
      'id'          => 'ci_textarea[' . $variation->ID . ']', 
      'label'       => __( 'My Textarea', 'woocommerce' ), 
      'placeholder' => '', 
      'description' => __( 'Enter the custom value here.', 'woocommerce' ),
      'value'       => get_post_meta( $variation->ID, 'ci_textarea', true ),
    )
  );
  // Select
  woocommerce_wp_select( 
  array( 
    'id'          => 'ci_select[' . $variation->ID . ']', 
    'label'       => __( 'My Select Field', 'woocommerce' ), 
    'description' => __( 'Choose a value.', 'woocommerce' ),
    'value'       => get_post_meta( $variation->ID, 'ci_select', true ),
    'options' => array(
      'one'   => __( 'CI Option 1', 'woocommerce' ),
      'two'   => __( 'CI Option 2', 'woocommerce' ),
      'three' => __( 'CI Option 3', 'woocommerce' )
      )
    )
  );
  // Checkbox
  woocommerce_wp_checkbox( 
  array( 
    'id'            => 'ci_checkbox[' . $variation->ID . ']', 
    'label'         => __('My Checkbox Field', 'woocommerce' ), 
    'description'   => __( 'Check me!', 'woocommerce' ),
    'value'         => get_post_meta( $variation->ID, 'ci_checkbox', true ), 
    )
  );
  // Hidden field
  woocommerce_wp_hidden_input(
  array( 
    'id'    => 'ci_hidden_field[' . $variation->ID . ']', 
    'value' => 'hidden_value'
    )
  );
}


// Save new fields for variations
function ci_save_variation_settings_fields( $post_id ) {
  // Text Field
  $text_field = $_POST['ci_text_field'][ $post_id ];
  if( ! empty( $text_field ) ) {
    update_post_meta( $post_id, 'ci_text_field', esc_attr( $text_field ) );
  }
  
  // Number Field
  $number_field = $_POST['ci_number_field'][ $post_id ];
  if( ! empty( $number_field ) ) {
    update_post_meta( $post_id, 'ci_number_field', esc_attr( $number_field ) );
  }
  // Textarea
  $textarea = $_POST['ci_textarea'][ $post_id ];
  if( ! empty( $textarea ) ) {
    update_post_meta( $post_id, 'ci_textarea', esc_attr( $textarea ) );
  }
  
  // Select
  $select = $_POST['ci_select'][ $post_id ];
  if( ! empty( $select ) ) {
    update_post_meta( $post_id, 'ci_select', esc_attr( $select ) );
  }
  
  // Checkbox
  $checkbox = isset( $_POST['ci_checkbox'][ $post_id ] ) ? 'yes' : 'no';
  update_post_meta( $post_id, 'ci_checkbox', $checkbox );
  
  // Hidden field
  $hidden = $_POST['ci_hidden_field'][ $post_id ];
  if( ! empty( $hidden ) ) {
    update_post_meta( $post_id, 'ci_hidden_field', esc_attr( $hidden ) );
  }
}