Your variable is getting set each time you read from the file. So, the variable will only hold whatever was in the last 100 bytes of the file you are reading from. 🙂
Either concatenate the string, or add a temp variable like below:
while (!feof($fp)) {
$this->ar[5] .= fgets($fp, 100);
OR
while (!feof($fp)) {
$temp = fgets($fp, 100);
$this->ar[5] .= $temp;
John Cornett