Programmatically log in a WordPress user

programmatic_login.php

/**
 * Programmatically logs a user in
 * 
 * @param string $username
 * @return bool True if the login was successful; false if it wasn't
 */
function programmatic_login( $username ) {
	
	if ( is_user_logged_in() ) {
		wp_logout();
	}
	
	add_filter( 'authenticate', 'allow_programmatic_login', 10, 3 );	// hook in earlier than other callbacks to short-circuit them
	$user = wp_signon( array( 'user_login' => $username ) );
	remove_filter( 'authenticate', 'allow_programmatic_login', 10, 3 );
	
	if ( is_a( $user, 'WP_User' ) ) {
		wp_set_current_user( $user->ID, $user->user_login );
		
		if ( is_user_logged_in() ) {
			return true;
		}
	}

	return false;
}

/**
 * An 'authenticate' filter callback that authenticates the user using only the username.
 *
 * To avoid potential security vulnerabilities, this should only be used in the context of a programmatic login,
 * and unhooked immediately after it fires.
 * 
 * @param WP_User $user
 * @param string $username
 * @param string $password
 * @return bool|WP_User a WP_User object if the username matched an existing user, or false if it didn't
 */
function allow_programmatic_login( $user, $username, $password ) {
	return get_user_by( 'login', $username );
}