A part of the functions.php file that is used to log in or register user via AJAX. Part of Header AJAX login/register gist (2)

functions.php

<?php

/********* AJAX Login ***********/
function yourtheme_ajax_login_init(){

    wp_register_script('ajax-login-script', get_template_directory_uri() . '/js/ajax-login-script.js', array('jquery') );
    wp_enqueue_script('ajax-login-script');

    wp_localize_script( 'ajax-login-script', 'ajax_login_object', array(
        'ajaxurl' => admin_url( 'admin-ajax.php' ),
        'redirecturl' => home_url(),
        'loadingmessage' => esc_html__('Sending user info, please wait...', 'yourtheme')
    ));

    // Enable the user with no privileges to run ajax_login() in AJAX
    add_action( 'wp_ajax_nopriv_ajaxlogin', 'ajax_login' );
}

// Execute the action only if the user isn't logged in
if (!is_user_logged_in()) {
    add_action('init', 'yourtheme_ajax_login_init');
}

if (!function_exists('ajax_login')) {
	function ajax_login(){

	    // First check the nonce, if it fails the function will break
	    check_ajax_referer( 'ajax-login-nonce', 'security' );

	    // Nonce is checked, get the POST data and sign user on
	    $info = array();
	    $info['user_login'] = $_POST['username'];
	    $info['user_password'] = $_POST['password'];
	    $info['remember'] = true;

	    $user_signon = wp_signon( $info, false );
	    if ( is_wp_error($user_signon) ){
	        echo json_encode(array('loggedin'=>false, 'message'=> esc_html__('Wrong username or password.', 'yourtheme')));
	    } else {
	        echo json_encode(array('loggedin'=>true, 'message'=> esc_html__('Login successful, redirecting...', 'yourtheme')));
	    }

	    die();
	}
}

/********* AJAX Registration ***********/
add_action( 'wp_ajax_register_action', 'yourtheme_handle_registration' );
add_action( 'wp_ajax_nopriv_register_action', 'yourtheme_handle_registration' );

if (!function_exists('yourtheme_handle_registration')) {
	function yourtheme_handle_registration(){

		if( $_POST['action'] == 'register_action' ) {

			$error = '';

			$uname = trim( $_POST['username'] );
			$email = trim( $_POST['mail_id'] );
			$fname = trim( $_POST['firname'] );
			$lname = trim( $_POST['lasname'] );

			if( empty( $_POST['username'] ) )
				$error .= '<p class="error">'.esc_html__('Enter username', 'yourtheme').'</p>';

			if( empty( $_POST['mail_id'] ) )
				$error .= '<p class="error">'.esc_html_('Enter email', 'yourtheme').'</p>';
			elseif( !filter_var($email, FILTER_VALIDATE_EMAIL) )
				$error .= '<p class="error">'.esc_html__('Enter valid email', 'yourtheme').'</p>';

			if( empty( $_POST['firname'] ) )
				$error .= '<p class="error">'.esc_html__('Enter first name', 'yourtheme').'</p>';
			elseif( !preg_match("/^[a-zA-Z'-]+$/",$fname) )
				$error .= '<p class="error">'.esc_html__('Enter valid first name', 'yourtheme').'</p>';

			if( empty( $_POST['lasname'] ) )
				$error .= '<p class="error">'.esc_html__('Enter last name', 'yourtheme').'</p>';
			elseif( !preg_match("/^[a-zA-Z'-]+$/",$lname) )
				$error .= '<p class="error">'.esc_html__('Enter valid last name', 'yourtheme').'</p>';

			if( empty( $error ) ){
				$status = wp_create_user( $uname, $email );
				if( is_wp_error($status) ){
					$msg = '';
					foreach( $status->errors as $key=>$val ){
						foreach( $val as $k=>$v ){
							$msg = '<p class="error">'.$v.'</p>';
						}
					}
					echo $msg;
				} else{
					$msg = '<p class="success">'.esc_html__('Registration successful!', 'yourtheme').'</p>';
					echo $msg;
				}
			} else{
				echo $error;
			}

			die(1);
		}
	}
}

add_action( 'user_register', 'yourtheme_register_user_metadata' );
add_action( 'profile_update', 'yourtheme_register_user_metadata' );

if (!function_exists('yourtheme_register_user_metadata')) {
	function yourtheme_register_user_metadata( $user_id ){

		if( !empty( $_POST['firname'] ) && !empty( $_POST['lasname'] ) && !empty( $_POST['username'] ) && !empty( $_POST['mail_id'] ) ){
			update_user_meta( $user_id, 'first_name', trim($_POST['firname']) );
			update_user_meta( $user_id, 'last_name', trim($_POST['lasname']) );
			update_user_meta( $user_id, 'username', trim($_POST['username']) );
			update_user_meta( $user_id, 'mail_id', trim($_POST['mail_id']) );
		}

		update_user_meta( $user_id, 'show_admin_bar_front', false );
	}
}