About sleep() and flush

programming
1 

<?php

for($i=1;$i<=10;$i++){
   
     echo $i.";";
     flush();
     sleep(2);

}

?>

Every 2 seconds print to screen 1 number
When i tested, it was ok
But some friends said that it waited for 20 seconds and then it printed from 1 --> 10
Their browsers have a problem ???