Notes

float Kp=0.5,Ki=0.1,Kd=2;
float error=0, P=0, I=0, D=0, PID_value=0;
float previous_error=0, previous_I=0;
int sensor[5]={0, 0, 0, 0, 0};
int initial_motor_speed=70;

void read_sensor_values(void);
void calculate_pid(void);
void motor_control(void);

void setup()
{
pinMode(9,OUTPUT); //PWM Pin 1
pinMode(10,OUTPUT); //PWM Pin 2
pinMode(4,OUTPUT); //Left Motor Pin 1
pinMode(5,OUTPUT); //Left Motor Pin 20
pinMode(2,OUTPUT); //Right Motor Pin 1
pinMode(3,OUTPUT); //Right Motor Pin 2
Serial.begin(9600); //Enable Serial Communications
}

void loop()
{
read_sensor_values();
calculate_pid();
motor_control();
}

void read_sensor_values()
{
sensor[0]=digitalRead(A0);
sensor[1]=digitalRead(A1);
sensor[2]=digitalRead(A2);
sensor[3]=digitalRead(A3);
sensor[4]=digitalRead(A4);

if((sensor[0]==0)&&(sensor[1]==0)&&(sensor[2]==0)&&(sensor[4]==0)&&(sensor[4]==1))
error=4;
else if((sensor[0]==0)&&(sensor[1]==0)&&(sensor[2]==0)&&(sensor[4]==1)&&(sensor[4]==1))
error=3;
else if((sensor[0]==0)&&(sensor[1]==0)&&(sensor[2]==0)&&(sensor[4]==1)&&(sensor[4]==0))
error=2;
else if((sensor[0]==0)&&(sensor[1]==0)&&(sensor[2]==1)&&(sensor[4]==1)&&(sensor[4]==0))
error=1;
else if((sensor[0]==0)&&(sensor[1]==0)&&(sensor[2]==1)&&(sensor[4]==0)&&(sensor[4]==0))
error=0;
else if((sensor[0]==0)&&(sensor[1]==1)&&(sensor[2]==1)&&(sensor[4]==0)&&(sensor[4]==0))
error=-1;
else if((sensor[0]==0)&&(sensor[1]==1)&&(sensor[2]==0)&&(sensor[4]==0)&&(sensor[4]==0))
error=-2;
else if((sensor[0]==1)&&(sensor[1]==1)&&(sensor[2]==0)&&(sensor[4]==0)&&(sensor[4]==0))
error=-3;
else if((sensor[0]==1)&&(sensor[1]==0)&&(sensor[2]==0)&&(sensor[4]==0)&&(sensor[4]==0))
error=-4;
else if((sensor[0]==0)&&(sensor[1]==0)&&(sensor[2]==0)&&(sensor[4]==0)&&(sensor[4]==0))
error=-5;


}

void calculate_pid()
{
P = error;
I = I + previous_I;
D = error-previous_error;

PID_value = (Kp*P) + (Ki*I) + (Kd*D);

previous_I=I;
previous_error=error;
}

void motor_control()
{
// Calculating the effective motor speed:
int left_motor_speed = initial_motor_speed-PID_value;
int right_motor_speed = initial_motor_speed+PID_value;

// The motor speed should not exceed the max PWM value
constrain(left_motor_speed,0,255);
constrain(right_motor_speed,0,255);

analogWrite(9,initial_motor_speed-PID_value); //Left Motor Speed
analogWrite(10,initial_motor_speed+PID_value); //Right Motor Speed
//following lines of code are to make the bot move forward
/*The pin numbers and high, low values might be different
depending on your connections */
digitalWrite(4,HIGH);
digitalWrite(5,LOW);
digitalWrite(2,HIGH);
digitalWrite(3,LOW);
}