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/**
 * 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 );
}