How to make a player only move in 1 direction (Unity) - unity3d

Question

I am trying to create a game in Unity where the player can only move in the direction it is facing, but the following code allows the player to move in all 4 directions.
(This is for a 3D project)
Any help would be appreciated! Thanks!
public class PlayerController : MonoBehaviour {
public float speed;
private Rigidbody rb;
void Start() {
rb = GetComponent<Rigidbody>();
}
void FixedUpdate() {
float moveHorizontal = Input.GetAxis("Horizontal");
float moveVertical = Input.GetAxis("Vertical");
Vector3 movement = new Vector3(moveHorizontal, 0.0f, moveVertical);
rb.AddForce(movement * speed);
}
}

Answer 1

So I didn't get what you wanted: first you said you wanted it to move only forward, then the only thing you have to do is get when he pressed a key and move forward while is not unpressed.
If you wanted to say that it can move all directions BUT only one at a time, the you will have to put the same code but with some changes:
First of all to make it move forward you have to get the forward of the transform, otherwise it will move in the same direction if you rotate it (you don't want that, no?).
Vector3 moveDirection = (transform.forward * Input.GetAxis("Vertical") + transform.right * Input.GetAxis("Horizontal")).normalized;
moveDirection.y = 0;
rb.velocity = moveDirection;
Then, to make that it ONLY moves to one direction at a time, you have to put the priority of the greatest axis number and if it's equal then you should think if you want to move forward or right (with it's axis value).

Answer 2

From the code you posted, I'm not sure where you are storing the player's facing-direction. However, I presume that it is stored as a Quaternion. If you have a player rotation quaternion called playerRotation, then you could do this (warning - untested):
Vector3 input = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
Vector3 normal = playerRotation * Vector3.forward;
Vector3 movement = Vector3.Dot(normal, input) * input;
If the game is first-person, then you can take a shortcut and just use Camera.current.transform.forward instead of the normal vector.
This will project the input direction onto the normal with the player's facing direction so that your movement force can only be in that direction.