How do i move my character in a direction that is not based on the character facing direction?

Example: the player is looking forward, if he press right the character goes on the right, but when he turns(180_degrees) himself if he press right the character turn left because the direction is based on where is he looking, how do I solve this?? PLS HELP!!! Here is the script: public class Player : MonoBehaviour { //Var public float movementSpeed; public GameObject camera; public Gun theGun;

 //Methods

 void Update()
 {
     //Player segue mouse
     Plane playerPlane = new Plane (Vector3.up, transform.position);
     Ray ray = UnityEngine.Camera.main.ScreenPointToRay (Input.mousePosition);
     float hitDist = 0.0f;

     if(playerPlane.Raycast(ray, out hitDist))
     {
         Vector3 targetPoint = ray.GetPoint(hitDist);
         Quaternion targetRotation = Quaternion.LookRotation (targetPoint - transform.position);
         targetRotation.x = 0;
         targetRotation.z = 0;
         transform.rotation = Quaternion.Slerp (transform.rotation, targetRotation, 7f * Time.deltaTime);
     }
     //movimenti Player
     if (Input.GetKey (KeyCode.W))
     {
         transform.Translate (Vector3.forward * movementSpeed * Time.deltaTime);


     }
     if (Input.GetKey (KeyCode.S))
     {
         transform.Translate (Vector3.back * movementSpeed * Time.deltaTime);


     }
     if (Input.GetKey (KeyCode.A))
     {
         transform.Translate (Vector3.left * movementSpeed * Time.deltaTime);


     }
     if (Input.GetKey (KeyCode.D))
     {
         transform.Translate (Vector3.right * movementSpeed * Time.deltaTime);

     }
     if (Input.GetMouseButtonDown(0))
         {
         theGun.isFiring = true;

         }
     if (Input.GetMouseButtonUp(0))
     {
         theGun.isFiring = false;
     }
     }