Save custom post metadata in WP

custom-post-metadata-save.php

<?php

/**
 * Save custom post metadata fields in WordPress.
 */
function my_custom_save_post(){
    global $post;

    // Populate this array with the names of your fields.
    // Should match ID/name attributes in your form.
    $customFields = array('field_name_one','field_name_two','field_name_three');

    // Loop through each field, check if it exists in the $_POST when saving the post, then
    // use update_post_meta() to update the metadata field for the blog post.
    foreach ($customFields as $field) {
        if (array_key_exists($field, $_POST)) {
            update_post_meta($post->ID, $field, $_POST[$field]);
        }
    }
}

add_action( 'save_post', 'my_custom_save_post');

Anyone using this should sanitize the $_POST data, which is not currently being done here.

You can learn more about data sanitization here:
https://developer.wordpress.org/themes/theme-security/data-sanitization-escaping/