Add card to customer account in Stripe

stripe.php

<?php
// Function to add a new card for your customer.
function sd_process_add_customer_card(){
	$redirect = false;

	if ( isset( $_POST['action'] ) && $_POST['action'] == 'add_customer_card' && wp_verify_nonce( $_POST['stripe_nonce'], 'stripe-nonce' ) ){
		if ( !isset( $_POST['card_number'] ) || !isset( $_POST['card_cvc'] ) || !isset( $_POST['card_exp_month'] ) || !isset( $_POST['card_exp_year'] ) ){
			$redirect = add_query_arg( array( 
				'card' => 'not-created',
			), $_POST['redirect'] );
		} else {
			global $stripe_options; // this variable includes your Stripe API keys and whether or not you are using test mode
			if ( !class_exists( 'Stripe' ) )
				require_once( //PATH TO Stripe.php );
				
			// check if we are using test mode
			if ( isset( $stripe_options['test_mode'] ) && $stripe_options['test_mode'] ) {
				$secret_key = trim( $stripe_options['test_secret_key'] );
			} else {
				$secret_key = trim( $stripe_options['live_secret_key'] );
			}

			\Stripe\Stripe::setApiKey( $secret_key );

			$card = \Stripe\Token::create(array(
				'card' => array(
					'number'	=> $_POST['card_number'],
					'exp_month'	=> $_POST['card_exp_month'],
					'exp_year'	=> $_POST['card_exp_year'],
					'cvc'		=> $_POST['card_cvc'],
				)
			));
			if ( $card ){
				$customer = \Stripe\Customer::retrieve( $_POST['customer_id'] );
				$customer->sources->create(array(
					'source'	=> $card
				));
				$redirect = add_query_arg( array( 
					'card' => 'added',
				), $_POST['redirect'] );
			}
		}
		if ($redirect) {
			wp_redirect( $redirect ); exit;
		}
	}
}
add_action( 'init', 'sd_process_add_customer_card' );
        
function sd_form_add_new_card(){
  if( !is_user_logged_in() ){
		return false;
  } else {
	  // retrieve customer id from user meta if it exists
    	$customer_id = get_user_meta( get_current_user_id(), '_stripe_customer_id', true );
	// if customer id doesn't exist create a new customer
	$stripe_customer_id = $customer_id ? $customer_id : sd_create_customer();
      	$display = '<div id="add-new-card">' . __( 'Add a New Card' ) . '</div>
          <form action="" method="POST" id="add-customer-card">
          <label>' . __('Card Number') . '</label>
          <input type="text" size="20" autocomplete="off" name="card_number" class="card-number"/>
          <label>' . __('CVC') . '</label>
          <input type="text" size="4" autocomplete="off" name="card_cvc" class="card-cvc"/>
          <label>' . __('Expiration (MM/YYYY)') . '</label>
          <input type="text" size="2" name="card_exp_month" class="card-expiry-month"/>
          <span> / </span>
          <input type="text" size="4" name="card_exp_year" class="card-expiry-year"/>
          <input type="hidden" name="redirect" value="' . get_permalink() . '"/>
          <input type="hidden" name="stripe_nonce" value="' . wp_create_nonce('stripe-nonce') . '"/>
          <input type="hidden" name="action" value="add_customer_card" />
          <input type="hidden" name="customer_id" value="' . $stripe_customer_id . '" />
          <button type="submit" id="stripe-submit">' . __('Add Card') . '</button>
          </form>';
      return $display;
    }
  }
}
add_shortcode( 'add_customer_card', 'sd_form_add_new_card' );
				
function sd_create_customer(){
	global $stripe_options; // this variable includes your Stripe API keys and whether or not you are using test mode
	if ( !class_exists( 'Stripe' ) )
		require_once( //PATH TO Stripe.php );

	// check if we are using test mode
	if ( isset( $stripe_options['test_mode'] ) && $stripe_options['test_mode'] ) {
		$secret_key = trim( $stripe_options['test_secret_key'] );
	} else {
		$secret_key = trim( $stripe_options['live_secret_key'] );
	}

	\Stripe\Stripe::setApiKey( $secret_key );
	// create a brand new customer
	$customer = \Stripe\Customer::create( array(
			'description' => 'Some description for my customer',
		)
	);
	if ( is_user_logged_in () ) {
		update_user_meta( get_current_user_id(), '_stripe_customer_id', $customer->id );
	}
	return $customer->id;
}