Foreign key empty (no data) PHP

I have a query regarding database and forms in PHP:

I have the following user table

CREATE TABLE IF NOT EXISTS 'login'.'usuarios' (

'nombres' VARCHAR(30) NULL,

'apellidos' VARCHAR(30) NULL,

'dni' VARCHAR(13) NOT NULL,

'telefono' VARCHAR(15) NULL,

'direccion' VARCHAR(45) NULL,

'sexo' VARCHAR(10) NULL,

'sesion_id' INT(10) NOT NULL,

PRIMARY KEY ('cedula'),

INDEX 'fk_usuarios_sesion_idx' ('sesion_id' ASC),

CONSTRAINT 'fk_usuarios_sesion'

FOREIGN KEY ('sesion_id')

REFERENCES 'login'.'sesion' ('id')

ON DELETE CASCADE

ON UPDATE CASCADE)
ENGINE = InnoDB;

And I have the table session

CREATE TABLE IF NOT EXISTS 'login'.'sesion' (

'id' INT(10) NOT NULL AUTO_INCREMENT,

'correo' VARCHAR(45) NULL,

'usuario' VARCHAR(10) NULL,

'password' VARCHAR(100) NULL,

'last_session' DATETIME NULL,

'activacion' INT NULL,

'token' VARCHAR(40) NULL,

'token_password' VARCHAR(100) NULL,

'password_request' INT(11) NULL,

'id_tipo' INT(11) NULL,

'estatus' INT(2) NULL,

PRIMARY KEY ('id'))
ENGINE = InnoDB;

They are related to each other from 1 to 1. The main session key is as a foreign key in users.

In PHP I have a form for users to register in my system. Everything is inserted normal in the tables but I only have one problem, because if in the table sessions the ID is autoincrement because being itself foreign in users does not fill too?

At this moment I have already registered several users in my system, and everything is inserted well, but the foreign key is always empty, it is not associated with the number that is assigned autoincrement in sessions.

To insert it I do it in the following way:

function registraUsuario($nombres, $apellidos, $cedula, $telefono, $direccion, $sexo){

            global $mysqli;

            $stmtr= $mysqli->prepare("INSERT INTO usuarios (nombres, apellidos, cedula, telefono, direccion, sexo) VALUES(?,?,?,?,?,?)");
            $stmtr->bind_param('ssssss', $nombres, $apellidos, $cedula, $telefono, $direccion, $sexo);

                if ($stmtr->execute()){
                return true;
                } else {
                return 0;

            }   

            }


        function registraSesion($email, $usuario, $pass_hash, $activo, $token, $tipo_usuario, $estatus){

            global $mysqli;

            $stmt = $mysqli->prepare("INSERT INTO sesion (correo, usuario, password, activacion, token, id_tipo, estatus) VALUES(?,?,?,?,?,?,?)");
            $stmt->bind_param('sssisii', $email, $usuario, $pass_hash, $activo, $token, $tipo_usuario, $estatus);


            if ($stmt->execute()){
                return $mysqli->insert_id;
                } else {
                return 0;   

            }   

        }

Additionally I must say that I have the foreign key as NULL because if I do not place it NULL, simply the data that I register for users (names, surnames, IDs ...) are not registered ... but being NULL if all is registered No inconvenience except the foreign key.

Please, can you help me with this problem that I have, or can you tell me how I can do or what I'm doing wrong?