Maybe a bit less ugly?
I changed it from 5000 to 79 so that one can view the result.

$window=79;
$i=-1;
$str="X" x 723;

substr($str,$i,0)="\n" while (($i+=$window+1) < length($str));

print $str;
[download]

This works by inserting "\n" at every $window+1 position.