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do-while

do-while 和 while 循环非常相似，区别在于表达式的值是在每次循环结束时检查而不是开始时。和正规的 while 循环主要的区别是 do-while 的循环语句保证会执行一次（表达式的真值在每次循环结束后检查），然而在正规的 while 循环中就不一定了（表达式真值在循环开始时检查，如果一开始就为 FALSE 则整个循环立即终止）。

do-while 循环只有一种语法：

<?php
$i = 0;
do {
   echo $i;
} while ($i > 0);
?>

以上循环将正好运行一次，因为经过第一次循环后，当检查表达式的真值时，其值为 FALSE（$i 不大于 0）而导致循环终止。

资深的 C 语言用户可能熟悉另一种不同的 do-while 循环用法，把语句放在 do-while(0) 之中，在循环内部用 break 语句来结束执行循环。以下代码片段示范了此方法：

<?php
do {
    if ($i < 5) {
        echo "i is not big enough";
        break;
    }
    $i *= $factor;
    if ($i < $minimum_limit) {
        break;
    }
    echo "i is ok";

    /* process i */

} while(0);
?>

如果还不能立刻理解也不用担心。即使不用此“特性”也照样可以写出强大的代码来。

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PHP手册 - N: do-while

用户评论:

david dot schueler at tel-billig dot de (18-Mar-2011 01:12)

If you are trying to use a construct like this:
<?php
do {
    // some code to run only one more time if expression is true
    if ($your_expression == true)
        continue;
} while (false);
?>
It will NOT loop as expected, because the continue tries to run the "next" loop, but the expression says just "false" so there is no next loop. The continue exits the while loop.
To get around this you may use an other expression, like this
<?php
do {
    // some code to run only one more time if expression is true
    if ($your_expression == true)
        continue;
    break;
} while (true);
?>
or use the goto statement since PHP 5.3.

xiaomao5 at live dot com (13-Mar-2011 03:48)

Another hack
If you want $type to only have a value of 0 or 1, you can do this:

<?php
do {
    echo 'Choose a type, 0 or 1: ';
    $type = trim(fgets(STDIN));
    if ($type == 0) {
        /* do stuff */
    } elseif ($type == 1) {
        /* do stuff */   
    }
} while ($type != 0 && $type != 1);
?>

shaida dot mca at gmail dot com (19-Aug-2010 07:01)

Example of Do while :-

<?php
$i = 0;
echo 'This code will run at least once because i default value is 0.<br/>';
do {
echo 'i value is ' . $i . ', so code block will run. <br/>';
++$i;
} while ($i < 10);
?>

andrew at NOSPAM dot devohive dot com (15-Sep-2008 06:44)

I'm guilty of writing constructs without curly braces sometimes... writing the do--while seemed a bit odd without the curly braces ({ and }), but just so everyone is aware of how this is written with a do--while...

a normal while:
<?php
   while ( $isValid ) $isValid = doSomething($input);
?>

a do--while:
<?php
   do $isValid = doSomething($input);
   while ( $isValid );
?>

Also, a practical example of when to use a do--while when a simple while just won't do (lol)... copying multiple 2nd level nodes from one document to another using the DOM XML extension

<?php
   # open up/create the documents and grab the root element
   $fileDoc  = domxml_open_file('example.xml'); // existing xml we want to copy
   $fileRoot = $fileDoc->document_element();
   $newDoc   = domxml_new_doc('1.0'); // new document we want to copy to
   $newRoot  = $newDoc->create_element('rootnode');
   $newRoot  = $newDoc->append_child($newRoot); // this is the node we want to copy to

   # loop through nodes and clone (using deep)
   $child = $fileRoot->first_child(); // first_child must be called once and can only be called once
   do $newRoot->append_child($child->clone_node(true)); // do first, so that the result from first_child is appended
   while ( $child = $child->next_sibling() ); // we have to use next_sibling for everything after first_child
?>

Ryan (18-Apr-2008 06:59)

I've found that the most useful thing to use do-while loops for is multiple checks of file existence. The guaranteed iteration means that it will check through at least once, which I had trouble with using a simple "while" loop because it never incremented at the end.

My code was:

<?php
$filename = explode(".", $_FILES['file']['name']); // File being uploaded
$i=0; // Number of times processed (number to add at the end of the filename)
do {
  /* Since most files being uploaded don't end with a number,
      we have to make sure that there is a number at the end
      of the filename before we start simply incrementing. I
      admit there is probably an easier way to do this, but this
      was a quick slap-together job for a friend, and I find it
      works just fine. So, the first part "if($i > 0) ..." says that
      if the loop has already been run at least once, then there
      is now a number at the end of the filename and we can
      simply increment that. Otherwise, we have to place a
      number at the end of the filename, which is where $i
      comes in even handier */

  if($i > 0) $filename[0]++;
  else $filename[0] = $filename[0].$i;
  $i++;
} while(file_exists("uploaded/".$filename[0].".".$filename[1]));

/* Now that everything is uploaded, we should move it
    somewhere it can be accessed. Hence, the "uploaded"
    folder. */
move_uploaded_file($_FILES['file']['tmp_name'], "uploaded/".$filename[0].".".$filename[1]);
?>

I'm sure there are plenty of ways of doing this without using the do-while loop, but I managed to toss this one together in no-time flat, and I'm not a great PHP programmer. =) It's simple and effective, and I personally think it works better than any "for" or "while" loop that I've seen that does the same thing.

jantsch at gmail dot com (29-Nov-2007 01:56)

Useful when you want to continue to read a recordset that was already being read like in:

<?
$sql = "select * from customers";
$res = mysql_query( $sql );

// read the first record
if( $rs = mysql_fetch_row( $res ) ){
   // do something with this record

}

// do another stuff here

// keep reading till the end
if( mysql_num_rows( $res )>1 ){
   do{
      // processing the records till the end

   }while( $rs = mysql_fetch_row( $res ));

}

?>

jayreardon at gmail dot com (10-Apr-2007 11:36)

There is one major difference you should be aware of when using the do--while loop vs. using a simple while loop:  And that is when the check condition is made. 

In a do--while loop, the test condition evaluation is at the end of the loop.  This means that the code inside of the loop will iterate once through before the condition is ever evaluated.  This is ideal for tasks that need to execute once before a test is made to continue, such as test that is dependant upon the results of the loop. 

Conversely, a plain while loop evaluates the test condition at the begining of the loop before any execution in the loop block is ever made. If for some reason your test condition evaluates to false at the very start of the loop, none of the code inside your loop will be executed.