save-post-meta-helper.php

<?php
/**
 * Saves post meta value
 *
 * @param int ID of the post
 * @param string Name of the post_meta key (same as the $_POST key and nonce name)
 * @param string Name of the nonce key
 * @param mixed The default value to be assigned if not set
 *
 * @return string Value that was saved
 */
function my_save_option( $post_id, $option_name, $nonce_name, $default_value = '', $sanitize_callback = 'sanitize_text_field' ) {
	
	// Grab the value from the $_POST object, or fallback to the default value
	$value = isset( $_POST[$option_name] ) ? $_POST[$option_name] : $default_value;

	if ( ! is_callable( $sanitize_callback ) )
		$sanitize_callback = 'sanitize_text_field';

	if ( is_array( $value ) )
		$value = array_map( $sanitize_callback, $value );
	else
		$value = call_user_func( $sanitize_callback, $value );
	
	// If nonce wasn't posted, exit out
	if( ! isset( $_POST[$nonce_name] ) )
		return;
	
	// Verify the nonce
	if( ! wp_verify_nonce( $_POST[$nonce_name], $option_name ) )
		return;
	
	// If we have a valid value, save it, else delete
	if( $value )
		update_post_meta( $post_id, $option_name, $value );
	else
		delete_post_meta( $post_id, $option_name );
	
	return $value;
}

function my_save_post( $post_id ) {
	
	// Exit out if we're autosaving
	if( defined('DOING_AUTOSAVE') && DOING_AUTOSAVE )
		return;
	
	// Exit out if user isn't allowed here
	if( !current_user_can('edit_post', $post_id) )
		return;
	
	my_save_option( $post_id, 'my_custom_field', 'my_custom_field_nonce' );
	
}

add_action( 'save_post', 'my_save_post' );