php获取最大值（accNumber），然后递增1

I'm trying to get the highest number and then increment it by 1. I get blank page, here is my code

   $query = $pdo->prepare("SELECT max(accnum) FROM accounts");
        $query->execute();

       while($num = $query->fetchAll()){

        if($num[1] == null){

        $accnumber = "100001";

        }else{

        $accnumber = $num[1]++; 
    }
        echo $accnumber;
    }   

what am i doing wrong here. Thanks

  $query = $pdo->prepare("SELECT max(accnum) FROM accounts");
        $query->execute();

       while($num = $query->fetchAll()){

        if($num[0] == null){

        $accnumber = "100001";

        }else{

        $accnumber = $num[0]++; 
    }
        echo $accnumber;
    }  

Maybe you need something like this ?

$query = $pdo->prepare("SELECT max(accnum)+1 as maximum FROM accounts");
$query->execute();
$result = $query->fetch(PDO::FETCH_ASSOC);
if($result['maximum'] == null) { 
    $result = "100001" 
} else { 
    $result = $accnumber['maximum'] 
}

You're using postincrement; that is, the ++ is after the variable, so it's not updated until after the assignment is complete.

Simply use this instead:

$accnumber = $num[1] + 1;

That way, $num isn't affected, and your value will be correct.

Since you're getting a blank page, this may not be the issue. So, you may also want to reconsider how you handle the values; you seem to be mixing strings and integers, and that probably won't end well. I'd suggest using only integers. Also, you should be using $num[0], since you're fetching the first value of a zero-based array:

$query = $pdo->prepare("SELECT max(accnum) FROM accounts");
$query->execute();

while($num = $query->fetchAll()){

    if($num[0] == null){

        $accnumber = 100001;
    }else{

        $accnumber = intval($num[0]) + 1;
    }
    echo $accnumber;
}