<?php
$point = 0;

 switch (true) {
    case $point >= 80:
        echo 'A';
        break;
    case $point >= 70:
        echo 'B';
        break;
    case $point >= 50:
        echo 'C';
        break;
    case $point >= 30:
        echo 'D';
        break;
    case $point >= 0:
        echo 'E';
        break;
    default:
        echo 'F';
        break;
 }