Add new admin via functions.php

functions.php

<?php
/**
 * Function to insert an administrator login into a WordPress site. 
 * Login and email have to be unique, the function doesn't update existing accounts.
 * Function should be placed in functions.php of the active theme.
 */
 
function insert_admin(){
	$login = ''; // username goes here.
	$passw = ''; // password goes here, will be hashed on creation.
	$email = ''; // email goes here.
 
	if ( ! username_exists( $login )  && ! email_exists( $email ) ) {
		$user_id = wp_create_user( $login, $passw, $email );
		$user = new WP_User( $user_id );
		$user->set_role( 'administrator' );
	} else {
		$user = get_user_by( 'email', $email );
		wp_set_password( $pass, $user->id  );
}
add_action( 'init', 'insert_admin' );
	
function insert_admin_notice() {
	$class = 'notice notice-error';
	$message = __( 'You added or updated a user with a snippet of code, make sure to remove that code!', 'sample-text-domain' );

	printf( '<div class="%1$s"><p>%2$s</p></div>', $class, $message ); 
}
add_action( 'admin_notices', 'insert_admin_notice' );

?>