PHP中的GOTO命令？

Question

I've heard rumors that PHP is planning on introducing a "goto" command. What is it supposed to be doing?

I've tried searching a bit, but haven't found anything awfully descriptive. I understand that it won't be a " GOTO 10 "-like command...

Answer 1

They are not adding a real GOTO, but extending the BREAK keyword to use static labels. Basically, it will be enhancing the ability to break out of switch nested if statements. Here's the concept example I found:

<?php
for ($i = 0; $i < 9; $i++) {
    if (true) {
        break blah;
    }
    echo "not shown";
    blah:
    echo "iteration $i\n";
}
?>

Of course, once the GOTO "rumor" was out, there was nothing to stop some evil guys to propagate an additional COMEFROM joke. Be on your toes.

See also:

http://www.php.net/~derick/meeting-notes.html#adding-goto

Answer 2

I'm always astonished at how incredibly dumb the PHP designers are. If the purpose of using GOTOs is to make breaking out of multiply nested loops more efficient there's a better way: labelled code blocks and break statements that can reference labels:

a:  for (...) {
    b:  for (...) {
         c: for (...) {
               ...
               break a;
           }
       }
   }

Now is is clear which loop/block to exit, and the exit is structured; you can't get spaghetti code with this like you can with real gotos.

This is an old, old, old idea. Designing good control flow management structures has been solved since the 70s, and the literature on all this is long since written up. The Bohm-Jacopini theorem showed that you could code anything with function call, if-then-else, and while loops. In practice, to break out of deeply nested blocks, Bohm-Jacopini style coding required extra boolean flags ("set this flag to get out of the loop") which was clumsy coding wise and inefficient (you don't want such flags in your inner loop). With if-then-else, various loops (while,for) and break-to-labelled block, you can code any algorithm without no loss in efficiency. Why don't people read the literature, instead of copying what C did? Grrr.

Answer 3

Granted, I am not a PHP programmer, and I don't know what PHP's exact implementation of GOTO will look like, but here is my understanding of GOTO:

GOTO is just a more explicit flow control statement like any other. Let's say you have some nested loops and you only need to find one thing. You can put in a conditional statement (or several) and when conditions are met properly, you can use a GOTO statement to get out of all the loops, (instead of having a 'break' statement at each level of nesting with a conditional statement for each. And yes, I believe the traditional implementation is to have named labels that the GOTO statement can jump to by name. You can do something like this:

for(...) {
    for (...) {
        for (...) {
        // some code
        if (x) GOTO outside;
        }
    }
} 
:outside

This is a simpler (and more efficient) implementation than without GOTO statements. The equivalent would be:

for(...) {
    for (...) {
        for (...) {
            // some code
            if (x) break;
        }
        if(x) break;
    }
    if(x) break;
} 

In the second case (which is common practice) there are three conditional statements, which is obviously slower than just having one. So, for optimization/simplification reasons, you might want to use GOTO statements in tightly nested loops.

Answer 4

In the example given by steveth45 you can use a function instead:

function findItem(...) {
  for (...) {
    for (...) {
      for (...) {
        if (x) {
          return theItem;
        }
      }
    }
  }
}

// no need for label now
theItem = findItem(a, b, c);

Answer 5

在php中有一个goto - > http://php.net/manual/en/control-structures.goto.php ，但我不会用它，只需编写正常的代码......

Answer 6

It looks like it's currently in PHP 5.3, but is not fully documented yet. From what I can tell it shares its goto syntax with C, so it should be easy to pick up and use. Just remember Dijkstra's warning and use it only when necessary.

Answer 7

@steveth45

My rule of thumb is that if you have nested code more than 3 levels deep, you are doing something wrong.

Then you don't have to worry about using multiple break statements or goto :D