PHP method to obtain x rules in aAAAxAAAa

As for this question, I just made a small modification to my program, and now the answer is out. Can any PHP expert please help me optimize my code structure? Thank you!
Original article: http://www.oschina.net/code/snippet_192190_16605

/**
 *
 * @author Leng Liuyun
 * @param Sina Weibo http://weibo.com/130326007
*/

function getLower($data){
 $length = strlen($data)-1;
 $str = '';
 for ($i =0;$i<$length;$i++){
 $flag = true;
 if(!isUpper($data[$i])){//The current lower case is true
 if($i == 3){
 $flag = getFlag($data,$i);
 //This is not true if the fourth last one is uppercase
 if(isUpper($data[$i+4])){
 $flag = false;
 }
 } else if($i == $length-3){
 $flag = getFlag($data,$i);
 //This is not true if the first fourth one is uppercase
 if(isUpper($data[$i-4] )){
 $flag = false;
 }
 }else if($i>3 && $i<$length-3){
 $flag = getFlag($data,$i);
 //If before || It is not true if the last fourth one is uppercase
 if(isUpper($data[$i+4]) || isUpper($data[$i-4])){
 $flag = false;
 }
 }else {
 $flag = false;
 }
 if($flag){
 @$str .= $data[$i];
 }
 }
 }
 return $str;
}
//The front of the public current character After three and three
function getFlag($data,$i){
 $flag = true;
 for($j=$i-3;$j<=$i+3;$j++){
 if($ j != $i){
 //This is not true if one of them is lowercase
 if(!isUpper($data[$j])){
 $flag = false;
 }
 }
 }
 return $flag;
 }
/**
 *
 * Determine whether it is a capital letter
*/
function isUpper($s){
 if(@ord($s) < 97){
 return true;
 }else{
 return false;
 }
}
$ res = '';
$handle = fopen("./input.txt",'r');
$d = fread($handle,filesize("./input.txt"));
$res = getlower (str_replace("rn","",$d));
/*while(!feof($handle)){//Line-by-line reading method
 $buffer = fgets($handle,4096);
 $res .= getlower($buffer);
}*/
echo $res;
fclose($handle);
?>