$postsubject=mysql_result($result,0,"post_text");
$array = explode("\n", $postsubject); /*Put each new line into an array */

for($i=0; $i<count($array); $i+=2) {
  echo $array[$i];
}

// better:
foreach($array as $key => $value) {
  if ($key%2 == 0) echo $value;
}