Store user details with PlayerPrefs

Hi guys!

I know that I can use PlayerPrefs to store user data but there are certain things bugging me. Basically, what I want to make is a "Remember me" feature. I'm working on uploading score to a webserver and I want it to be able to auto-complete all the text fields based on the username ONLY. I've looked into PlayerPrefsX but I can't get it to work because it requires Built-In Array and it's not possible to dynamically push a new username into the array. Since I can't limit how many usernames to be written into the PlayerPrefs, I can't simply specify the limit of the array. I've been looking for answers but couldn't find it. Thanks in advance =)

Edit: I've tried working on making "auto-complete" feature but I can't get it to work. I'm not quite sure if the PlayerPrefs is actually working here since I can't debug it on device because the trigger of PlayerPrefs is based on iPhoneKeyboard.done. Here is the script I've written so far:

 function RequestInput(){
     if(showGUI){
         if(keyboardUserName){
             userName = keyboardUserName.text;
             if(keyboardUserName.done){
                 CheckUserData(userName);
                 if(dataExist){
                     keyboardPersonName.text = ""+personName;
                     keyboardEmail.text = ""+emailAddress;
                     keyboardID.text = ""+icNumber;
                 }else if(!dataExist){
                     personName = keyboardPersonName.text;
                     emailAddress = keyboardEmail.text;
                     idNumber = keyboardID.text;
                 }
             }
         }
     }
 }
 
 function CheckUserData(name:String){
     var tempName:String = name;
     for(var i = 0; i < 100; i++){
         if(PlayerPrefs.HasKey(i+"Username")){
             if(PlayerPrefs.GetString(i+"Username") == tempName){
                 personName = PlayerPrefs.GetString(i+"Name");
                 emailAddress = PlayerPrefs.GetString(i+"Email");
                 idNumber = PlayerPrefs.GetString(i+"ID");
                 dataExist = true;
                 Debug.Log(dataExist);
             }
         }else{
             PlayerPrefs.SetString(i+"Username", userName);
             PlayerPrefs.SetString(i+"Name", personName);
             PlayerPrefs.SetString(i+"Email", emailAddress);
             PlayerPrefs.SetString(i+"IC", idNumber);
             Debug.Log("Username: "+userName +", Name: "+personName +", E-mail:"+emailAddress +", ID: "+idNumber);
             dataExist = false;
             Debug.Log(dataExist);
         }
     }
 }

RequestInput() function is called from function OnGUI() so it's always called after the score uploading screen comes up. Anything wrong in the script?