How to restart a counter in a program using a push button in a LDR Tripwire setup
Try out this code. I made some modification and also comment out that things.

#include &lt;LiquidCrystal.h&gt;

LiquidCrystal lcd(7, 6, 5, 4, 3);

int laser = 8;
int pushbutton = 12;
int flag = 0; 
int myCounter = 0; 

void setup() 
{ 
  pinMode(laser,OUTPUT);
  pinMode(pushbutton,INPUT_PULLUP); // Provide pull-up

  lcd.begin(16, 2); 
  lcd.print("Person Counter:");
  lcd.setCursor(0,1); 
  lcd.print(0); 
} 

void loop() 
{
  lcd.setCursor(0, 1);
  digitalWrite(laser,HIGH); 

  if(analogRead(A0) &lt; 600) //A0= LDR
  {
    flag = 1;
  } 

  if(analogRead(A0) &gt; 600 &amp;&amp; flag == 1) 
  {
    myCounter++; 
    delay(200); 
    lcd.print(myCounter); 
    delay(20); 
    flag = 0;
  }

  if(digitalRead(pushbutton) == LOW) // When button pressed
  {
    myCounter = 0; // Set counter to 0
  }
}