PHP

wp is user admin

if ( current_user_can('administrator') ) {} // also others than admin w same capabilities
if ( is_user_admin() ){} // if it is admin
if ( is_admin() ){} //if the view is not the admin interface but the theme

wordpress is admin

// Determines whether the current request is for an 
// administrative interface page.
is_admin()

user is admin wordpress

if( ! function_exists( 'current_user_has_role' ) ){
    function current_user_has_role( $role ){
        return user_has_role_by_user_id( get_current_user_id(), $role );
    }
}

if( ! function_exists( 'get_user_roles_by_user_id' ) ){
    function get_user_roles_by_user_id( $user_id ) {
        $user = get_userdata( $user_id );
        return empty( $user ) ? array() : $user->roles;
    }
}

if( ! function_exists( 'user_has_role_by_user_id' ) ){
    function user_has_role_by_user_id( $user_id, $role ) {

        $user_roles = get_user_roles_by_user_id( $user_id );

        if( is_array( $role ) ){
            return array_intersect( $role, $user_roles ) ? true : false;
        }

        return in_array( $role, $user_roles );
    }
}