The type or namespace name PlayerHealth could not be found (are you missing a using directive or an assembly reference

Hi so I’m just started using unity and saw a tutorial on youtube on how to make a zombie FPS and when I use PlayerHealth for an enemy attack it gives me the error in the title. Here’s my code,

using UnityEngine;
using System.Collections;

public class EnemyAttack : MonoBehaviour
{

    public float maxDistance;
    public float coolDownTimer;
    public PlayerHealth ph;
    public int damage;

    private Transform myTransform;
    public Transform target;

    // Use this for initialization
    void Start()
    {
        GameObject go = GameObject.FindGameObjectWithTag("Player");
        target = go.transform;
        myTransform = transform;
        maxDistance = 3;
        coolDownTimer = 0;
        damage = -10;

        ph = (PlayerHealth)go.GetComponent(typeof(PlayerHealth));
    }

    // Update is called once per frame
    void Update()
    {
        float distance = Vector3.Distance(target.position, myTransform.position);
        if (distance < maxDistance)
        {
            Attack();
        }

        if (coolDownTimer > 0)
        {
            coolDownTimer = coolDownTimer - Time.deltaTime;
        }

        if (coolDownTimer < 0)
        {
            coolDownTimer = 0;
        }
    }

    void Attack()
    {
        Vector3 dir = Vector3.Normalize(target.position - myTransform.position);
        float direction = Vector3.Dot(dir, transform.forward);
        if (direction > 0)
        {
            if (coolDownTimer == 0)
            {
                ph.ChangeHealth(damage);
                coolDownTimer = 2;
            }
        }
    }
}

First check if you have a class PlayerHealth. If you do: locate the file, check if the filename is PlayerHealth as well. If not change it to the class name. If after that the problem still occurs → check if the class is wrapped inside a namespace, something like this:

//This is the namespace name
namespace HealthClasses 
{
	public class PlayerHealth // Your class
	{
		//Whatever's inside your class
	}
}

If it is then you add ‘using (namespace name);’ at the top of the EnemyAttack class.
using HealthClasses in this example.

Also:

//instead of writing :
ph = (PlayerHealth)go.GetComponent(typeof(PlayerHealth));

//you can simply write this:
ph = go.GetComponent<PlayerHealth>();

It does the same thing.

Hope this helps!