อธิบายคำสั่ง PHP

<?php

$sql ="select * from student order by id asc ";

$query=mysql_query($sql) or die(mysql_error());

$num=mysql_num_rows($query);

echo $num;

?>

My if else

<?php
$score=63;
if($score < 50){
echo 'grade 0';
}else if ($score < 56){
echo 'grade 1';
}else if ($score < 60){
echo 'grade 1.5';
}else if ($score < 66){
echo 'grade 2';
}else if ($score < 70){
echo 'grade 2.5';
}else if ($score < 76){
echo 'grade 3';
}else if ($score < 80){
echo 'grade 3.5';
}else {echo'grade4';}
?>