Custom form submission using admin-post with wp_mail()

form-submit-admin-post.php

<!-- Sample custom form-->
<!-- We use admin-post.php provided to fully utilize the event driven nature of WordPress. -->
<form id="form-id" action="<?php echo esc_url( admin_url('admin-post.php') ); ?>" method="post">
  <label for="first-name">First Name</label>
  <input type="text" name="first-name" id="first-name" required>
  <label for="last-name">Last Name</label>
  <input type="text" name="last-name" id="last-name" required>
  <label for="email">Email Address</label>
  <input type="email" name="email" id="email" required>
  <label for="birthday">Birthday:</label>
  <input type="date" id="birthday" name="birthday">
  <!-- We also need to add the action hidden input so that we can trigger the more specific hook -->
  <input type="hidden" name="action" value="form_submit_action">
  <input type="submit" class"submit-form" value="Submit"/>
</form>


<?php 
// In your functions.php
// Action function
function form_submit_action() {
  // You can now use $_GET/$_POST variables depending on what method you used in your form
  // In this case we are using method post
  $first_name = sanitize_text_field($_POST['first-name']);
  $last_name = sanitize_text_field($_POST['last-name']);
  $email = sanitize_email($_POST['email']);
  $birthday = sanitize_text_field($_POST['birthday']);
 
  // Then do the processing here like create new post/user, update new post/user , etc.
  // But on this example im gonna show you how send an email, create your own custom html body format.
  
  // Send to admin
  $to = get_bloginfo('admin_email'); // or 'sendee@email.com' to specify email
  // Email subject
  $subject = 'Customer Wishlist';
  // Email body/content (tricky part)
  /* Instead of:
      $body = '<div>
                <p>'. $first_name .'</p>
               </div>'; 
   */
  // We can create a custom function with the post fields as your attributes
  $body = my_email_body_function($first_name,$last_name,$email,$birthday);
  $headers = array('Content-Type: text/html; charset=UTF-8');
  wp_mail( $to, $subject, $body, $headers );
  
  // Then redirect to desired page
  wp_redirect(home_url('/desired-page'));
  
}
// Necessary action hooks
// Use our specific action form_submit_action to process the data related to our request
add_action( 'admin_post_nopriv_form_submit_action', 'form_submit_action' );
add_action( 'admin_post_form_submit_action', 'form_submit_action' );

// Email body function declaration
function my_email_body_function($first_name,$last_name,$email,$birthday) {
  ob_start(); // We have to turn on output buffering. VERY IMPORTANT! or else wp_mail() wont work 
  // Then setup your email body using the postfields from the attritbutes passed on. ?>
  <table style="width:100%; border-collapse: collapse;">
  <tr>
    <th style="border: 1px solid #dddddd;text-align: left;padding: 8px;">Name:</th>
    <th style="border: 1px solid #dddddd;text-align: left;padding: 8px;"><?php echo $first_name; ?></th>
  </tr>
  <tr>
    <th style="border: 1px solid #dddddd;text-align: left;padding: 8px;">Last Name:</th>
    <th style="border: 1px solid #dddddd;text-align: left;padding: 8px;"><?php echo $last_name; ?></th>
  </tr>
  <tr>
    <th style="border: 1px solid #dddddd;text-align: left;padding: 8px;">Email:</th>
    <th style="border: 1px solid #dddddd;text-align: left;padding: 8px;"><?php echo $email; ?></th>
  </tr>
  <tr>
    <th style="border: 1px solid #dddddd;text-align: left;padding: 8px;">Birthday:</th>
    <th style="border: 1px solid #dddddd;text-align: left;padding: 8px;"><?php echo $birthday; ?></th>
  </tr>
  </table
  
  <?php 
  return ob_get_contents();
  ob_get_clean();
}