I've been using this PID controller code for years.

PID.cs

using UnityEngine;

// From http://forum.unity3d.com/threads/68390-PID-controller
// Thank you andeeeee
public class PID
{
    public float pFactor, iFactor, dFactor;

    float integral;
    float lastError;

    public PID()
    {
        pFactor = 0.0f;
        iFactor = 0.0f;
        dFactor = 0.0f;
    }

    public PID(Vector3 values)
    {
        pFactor = values.x;
        iFactor = values.y;
        dFactor = values.z;
    }

    public PID(float pFactor, float iFactor, float dFactor)
    {
        this.pFactor = pFactor;
        this.iFactor = iFactor;
        this.dFactor = dFactor;
    }

    public float Update(float setpoint, float actual, float timeFrame)
    {
        // Handle when time is paused.
        if (timeFrame == 0f)
            return actual;

        float present = setpoint - actual;
        integral += present * timeFrame;
        float deriv = (present - lastError) / timeFrame;
        lastError = present;
        return present * pFactor + integral * iFactor + deriv * dFactor;
    }

    public void SetPidValues(float p, float i, float d)
    {
        pFactor = p;
        iFactor = i;
        dFactor = d;
    }
    
    public void SetPidValues(Vector3 pidValues)
    {
        pFactor = pidValues.x;
        iFactor = pidValues.y;
        dFactor = pidValues.z;
    }
}