title: Custom placeholder for all WooCommerce checkout fields
body: <p>I am trying to add a placeholder to my WooCommerce checkout fields, and it's working perfectly for every field <strong>except for the phone and the email</strong> fields. </p>
  
  <p>This is the code I am using:</p>
  
  <pre><code>add_filter('woocommerce_default_address_fields', 'override_address_fields');
  function override_address_fields( $address_fields ) {
      $address_fields['first_name']['placeholder'] = 'Fornavn';
      $address_fields['last_name']['placeholder'] = 'Efternavn';
      $address_fields['address_1']['placeholder'] = 'Adresse';
      $address_fields['state']['placeholder'] = 'Stat';
      $address_fields['postcode']['placeholder'] = 'Postnummer';
      $address_fields['city']['placeholder'] = 'By';
      $address_fields['phone']['placeholder'] = 'Telefon';
      $address_fields['email']['placeholder'] = 'Email';
      return $address_fields;
  }
  </code></pre>
  
  <p>Where am I going wrong? Why isn't the phone and email putting out any results?</p>
  
  <p>I have taken the IDs from that two fields using my browser developer tool inspector.</p>
  
  <hr>
  
  <p><strong>Edit:</strong></p>
  
  <p><strong>I have also tried this suggested code:</strong></p>
  
  <pre><code>add_filter('woocommerce_checkout_fields', 'override_checkout_fields');
  function override_checkout_fields( $checkout_fields ) {
      $checkout_fields['first_name']['placeholder'] = 'Fornavn';
      $checkout_fields['last_name']['placeholder'] = 'Efternavn';
      $checkout_fields['address_1']['placeholder'] = 'Adresse';
      $checkout_fields['state']['placeholder'] = 'Stat';
      $checkout_fields['postcode']['placeholder'] = 'Postnummer';
      $checkout_fields['city']['placeholder'] = 'By';
      $checkout_fields['phone']['placeholder'] = 'Telefon';
      $checkout_fields['email']['placeholder'] = 'Email';
      return $checkout_fields;
  }
  </code></pre>
  
  <p><strong>And this one too:</strong></p>
  
  <pre><code>add_filter('woocommerce_checkout_fields', 'override_checkout_fields');
  function override_checkout_fields( $checkout_fields ) {
      $checkout_fields['billing_first_name']['placeholder'] = 'Fornavn';
      $checkout_fields['billing_last_name']['placeholder'] = 'Efternavn';
      $checkout_fields['billing_address_1']['placeholder'] = 'Adresse';
      $checkout_fields['billing_state']['placeholder'] = 'Stat';
      $checkout_fields['billing_postcode']['placeholder'] = 'Postnummer';
      $checkout_fields['billing_city']['placeholder'] = 'By';
      $checkout_fields['billing_phone']['placeholder'] = 'Telefon';
      $checkout_fields['billing_email']['placeholder'] = 'Email';
      return $checkout_fields;
  }
  </code></pre>
  
  <p>But it doesn't work.</p>