[PHP] print even numbers (intermediate coding challenge)

Why this challenge not pass when submitted?
QUestion:
Write a program to print the even numbers between 0 to n
Write a program to print the even numbers between 0 and n ( read from input ) in comma separated form.

Example input:
10
Example output:
0,2,4,6,8,10

my code:

<?php

$num = readline();

$res = '0';
for($i=1; $i <=$num; $i++){
  if($i % 2 == 0 ){
    $res .= ','.$i;
  }
}
echo $res;

?>

thanks