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// 1. Show custom input field above Add to Cart
 
add_action( 'woocommerce_before_add_to_cart_button', 'bbloomer_product_add_on', 9 );
 
function bbloomer_product_add_on() {
	if ( is_product() && has_term( array (
		'cordless-drills',
		'domino-joiners',
		'dust-extractors',
		'edge-banders',
		'jigsaws',
		'kapex-miter-saws',
		'mft-tables',
		'planers',
		'routers',
		'sanders',
		'track-saws'),
								  	'product_cat' ) )
		
	 {
    $value = isset( $_POST['_tshirt_add_on'] ) ? sanitize_text_field( $_POST['_tshirt_add_on'] ) : '';
    echo '<shirt><label>FREE T-Shirt with Power Tool<Br /> (please specify size M, L, XL, XXL) </label><p><input name="_tshirt_add_on" value="' . $value . '"></p></shirt>';
	}
}
 
// -----------------------------------------
// 2. Throw error if custom input field empty
 
add_filter( 'woocommerce_add_to_cart_validation', 'bbloomer_product_add_on_validation', 10, 3 );
 
function bbloomer_product_add_on_validation( $passed, $product_id, $qty ){
   if( isset( $_POST['_tshirt_add_on'] ) && sanitize_text_field( $_POST['_tshirt_add_on'] ) == '' ) {
      wc_add_notice( 'T-shirt is a required field', 'error' );
      $passed = false;
   }
    return $passed;
}
 
// -----------------------------------------
// 3. Save custom input field value into cart item data
 
add_filter( 'woocommerce_add_cart_item_data', 'bbloomer_product_add_on_cart_item_data', 10, 2 );
 
function bbloomer_product_add_on_cart_item_data( $cart_item, $product_id ){
    if( isset( $_POST['_tshirt_add_on'] ) ) {
        $cart_item['tshirt_add_on'] = sanitize_text_field( $_POST['_tshirt_add_on'] );
    }
    return $cart_item;
}
 
// -----------------------------------------
// OLD STEP #4. Display custom input field value @ Cart
 
//add_filter( 'woocommerce_get_item_data', 'bbloomer_product_add_on_display_cart', 10, 2 );
 
//function bbloomer_product_add_on_display_cart( $_data, $cart_item ) {
 //   if ( isset( $cart_item['tshirt_add_on'] ) ){
 //       $data[] = array(
  //          'name' => 'ShirtSize',
  //          'value' => 'T-Shirt:' . sanitize_text_field( $cart_item['tshirt_add_on'] )
  //      );
 //   }
 //   return $data;
// }



NEW STEP 4. STEP #4 IN WOOCOMMERCE > CART.PHP LINE 39 BELOW SKU :
 if ( isset( $cart_item['tshirt_add_on'] ) ) { echo "<span class='sku'>Shirt Size/Hat: ". $cart_item['tshirt_add_on'] . "</span>";}
 
// -----------------------------------------
// 5. Save custom input field value into order item meta
 
add_action( 'woocommerce_add_order_item_meta', 'bbloomer_product_add_on_order_item_meta', 10, 2 );
 
function bbloomer_product_add_on_order_item_meta( $item_id, $values ) {
    if ( ! empty( $values['tshirt_add_on'] ) ) {
        wc_add_order_item_meta( $item_id, 'T-Shirt Size', $values['tshirt_add_on'], true );
    }
}
 
// -----------------------------------------
// 6. Display custom input field value into order table
 
add_filter( 'woocommerce_order_item_product', 'bbloomer_product_add_on_display_order', 10, 2 );
 
function bbloomer_product_add_on_display_order( $cart_item, $order_item ){
    if( isset( $order_item['tshirt_add_on'] ) ){
        $cart_item_meta['tshirt_add_on'] = $order_item['tshirt_add_on'];
    }
    return $cart_item;
}
 
// -----------------------------------------
// 7. Display custom input field value into order emails
 
add_filter( 'woocommerce_email_order_meta_fields', 'bbloomer_product_add_on_display_emails' );
 
function bbloomer_product_add_on_display_emails( $fields ) { 
    $fields['tshirt_add_on'] = 'T-Shirt Size';
    return $fields; 
}