Im not sure if theres a better way to do this but here goes..:
$lines=file('http://www.site.com/index.htm');
for($i=29;$i<count($lines);$i++) {
$info.=$lines[$i];
}
But in my opinion, you should do the replace inside the for-loop:
$lines=file('http://www.site.com/index.htm');
for($i=29;$i<count($lines);$i++) {
$currline=str_replace('black','white',$lines[$i]);
$info.=$currline;
}