你为什么在PHP中有一个字符串> ='0'的条件？

Question

As part of an integration against a third party service I need to calculate a check digit. I have been given a PHP snippet to show the algorithm but I need to implement it in pl/sql. The PHP snippet (taken from here) is:

function arvutaViitenumber($nr){
     $nr = (string)$nr;
     $kaal = array(7,3,1);
     $sl = $st = strlen($nr);
     $total = 0;
     while($sl > 0 and substr($nr, --$sl, 1) >='0'){
         $total += substr($nr, ($st-1)-$sl, 1)*$kaal[($sl%3)];
     }
     $kontrollnr = ((ceil(($total/10))*10)-$total);
     return $nr.$kontrollnr;
}

I understand most of it except the substr($nr, --$sl, 1) >='0'condition in the loop. There are two things I do not understand about this:

1. The main body of the loop takes each digit in order from the start. Why then have a condition where the loop exits before processing the nth digit from the start based on the state of the nth digit from the end?

2. When comparing a single-character string as greater or equal string '0' can the condition ever be false? In my tests it is always true, even if the string compared is a character not a number.

Question 1 is more of accademic intrest. My real interest is question 2 and how I will implement this check in the pl/sql version (or if I even need to?)

Answer 1

1. Well, think how the reverse would go.

 $st = strlen($nr);
 $sl = $total = 0;
 while($sl < strlen($nr) and substr($nr, ++$sl, 1) >='0'){

Notice the strlen in the while loop - bad practice. Although in PHP it's fast, in most programming languages this is horrible for performance. The alternative would be to use another variable, which would be reduntant.

So the best option for both lazyness and performance is to start from the end.

2. As far as I can tell, the only purpose of that string comparison is to make sure the position in the ASCII table is higher or equal to '0' ASCII no. 48). First, check the http://www.asciitable.com/

Then take a look at the following tests I did:

var_dump(' ' >= '0'); // false // space has ASCII no 32
var_dump('(' >= '0'); // false // ASCII no 47
var_dump('A' >= '0'); // true // ASCII no 65
var_dump('[' >= '0'); // true // ASCII no 71

So it's almost an alphanumeric test, as @mudasobwa mentioned.