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for

for 循环是 PHP 中最复杂的循环结构。它的行为和 C 语言的相似。 for 循环的语法是： 

for (expr1; expr2; expr3)
    statement

第一个表达式（expr1）在循环开始前无条件求值一次。

expr2 在每次循环开始前求值。如果值为 TRUE，则继续循环，执行嵌套的循环语句。如果值为 FALSE，则终止循环。

expr3 在每次循环之后被求值（执行）。

每个表达式都可以为空或包括逗号分隔的多个表达式。表达式 expr2 中，所有用逗号分隔的表达式都会计算，但只取最后一个结果。expr2 为空意味着将无限循环下去（和 C 一样，PHP 认为其值为 TRUE）。这可能不像想象中那样没有用，因为经常会希望用 break 语句来结束循环而不是用 for 的表达式真值判断。

考虑以下的例子，它们都显示数字 1 到 10：

<?php
/* example 1 */

for ($i = 1; $i <= 10; $i++) {
    echo $i;
}

/* example 2 */

for ($i = 1; ; $i++) {
    if ($i > 10) {
        break;
    }
    echo $i;
}

/* example 3 */

$i = 1;
for (;;) {
    if ($i > 10) {
        break;
    }
    echo $i;
    $i++;
}

/* example 4 */

for ($i = 1, $j = 0; $i <= 10; $j += $i, print $i, $i++);
?>

当然，第一个例子看上去最正常（或者第四个），但用户可能会发现在 for 循环中用空的表达式在很多场合下会很方便。

PHP 也支持用冒号的 for 循环的替代语法。 

for (expr1; expr2; expr3):
    statement;
    ...
endfor;

我们经常需要对下面这样的数组进行遍历：

<?php
/*
 * 我们想要在遍历的过程中改变以下数组中某些元素的值
 */
$people = Array(
        Array('name' => 'Kalle', 'salt' => 856412), 
        Array('name' => 'Pierre', 'salt' => 215863)
        );

for($i = 0; $i < sizeof($people); ++$i)
{
    $people[$i]['salt'] = rand(000000, 999999);
}
?>

以上代码的问题，在于for的第二个表达式会导致代码执行很慢－－因为每次循环时都要 计算一遍数组的长度。由于数组的长度始终不变，我们可以用一个中间变量来存储数组长度， 然后用这个变量作为for循环的第二个表达式。这样在循环的时候就可以直接使用该变量的值， 不用每次重新计算。 如下：

<?php
$people = Array(
        Array('name' => 'Kalle', 'salt' => 856412), 
        Array('name' => 'Pierre', 'salt' => 215863)
        );

for($i = 0, $size = sizeof($people); $i < $size; ++$i)
{
    $people[$i]['salt'] = rand(000000, 999999);
}
?>

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PHP手册 - N: for

用户评论:

Anonymous (14-Feb-2012 11:49)

Regarding [kanirockz at gmail dot com 21-Mar-2010 11:48]; here's a more compact form of your program and for loop:

<?php

$lentxt=strlen($text="Welcome to PHP");
$searchchar="e";

for($i=0, $count=0; $i<$lentxt; substr($text,$i++,1)==$searchchar ? $count++ : NULL);

echo $count;

?>

Anonymous (14-Feb-2012 11:28)

Regarding [eduardofleury at uol dot com dot br 14-Jun-2007 06:18]; here's a slightly more compact form of your program (excluding the demonstration of $p and its use as a variable-by-reference):

<?php
//this is a different way to use the 'for'
//Essa é uma maneira diferente de usar o 'for'
for($i = $x = $z = 1; $i <= 10; $z = ++$i + $x+=2){
  
    print "\$i = $i , \$x = $x , \$z = $z <br />";
  
}

?>

matthiaz (08-Feb-2012 10:37)

Looping through letters is possible. I'm amazed at how few people know that.

for($col = 'R'; $col != 'AD'; $col++) {
    echo $col.' ';
}

returns: R S T U V W X Y Z AA AB AC

Take note that you can't use $col < 'AD'. It only works with !=
Very convenient when working with excel columns.

kanirockz at gmail dot com (21-Mar-2010 07:49)

Here is another simple example for " for loops"

<?php

$text="Welcome to PHP";
$searchchar="e";
$count="0"; //zero

for($i="0"; $i<strlen($text); $i=$i+1){
   
    if(substr($text,$i,1)==$searchchar){
   
       $count=$count+1;
    }

}

echo $count

?>

this will be count how many "e" characters in that text (Welcome to PHP)

kanirockz at gmail dot com (21-Mar-2010 07:48)

Here is another simple example for " for loops"

<?php

$text="Welcome to PHP";
$searchchar="e";
$count="0"; //zero

for($i="0"; $i<strlen($text); $i=$i+1){
   
    if(substr($text,$i,1)==$searchchar){
   
       $count=$count+1;
    }

}

echo $count

?>

this will be count how many "e" characters in that text (Welcome to PHP)

Steven (11-Jan-2009 06:50)

Alternating form rows:

<?php

$rows = 4;

echo '<table><tr>';

for($i = 0; $i < 10; $i++){
    echo '<td>' . $i . '</td>';
    if(($i + 1) % $rows == 0){
        echo '</tr><tr>';
    }
}

echo '</tr></table>';

?>

Changing $rows will change how many columns are in a row.

dkimbel13 at gmail dot com (07-Jan-2009 11:41)

Just a note on looping through an array using the for() loop.

with the array...
<?php $array = array("value1","value2","value3"); ?>

then...
<?php
for(reset($array),current($array),next($array){
    echo("Element ".key($array)." contains ".current($array)."<br/>";
}
?>

is the equivalent of...
<?php
for($i=0;$i<count($array);$i++){
    echo("Element $i contains $array[$i]<br/>");
}
?>

I don't know if there is any advantage, just thought I would mention it.

http://badluck.tv (24-Mar-2008 01:05)

Nested For Loop with the same iterator as the parent.
(Well formatted so the resulting code is clean when executed).
Useful for outputting a data array into a table, ie. images.

<?php
//Dummy data
$data = array(73,74,75,76,78,79,80,81,82,83,84,85,86,87);

//Our 'stepping' variable
$g = 0;

//Our rowcount
$rowcount = 0;

echo "<table cellspacing='0'>\r";
    for ($i=0; $i<count($data); ) {

        $rowcount++;
        echo "    <tr>\r"; //New row

        $g = $i + 3; //Set our nested limit
        for( ; $i<$g; $i++) { //nested for loop

            if (!isset($data[$i])) { //Allow us to break on incomplete rows
                break;
            }

            echo "        <td style='border: 1px #000 solid;'>\r"; //Out put a cell
            echo "            <p>Row $rowcount <br/> Cell: $i <br/> Data: $data[$i]</p>\r";
            echo "        </td>\r";
        }

        echo "    </tr> \r"; //End New Row
    }

echo "</table>\r";?>

eduardofleury at uol dot com dot br (14-Jun-2007 02:18)

<?php
//this is a different way to use the 'for'
//Essa é uma maneira diferente de usar o 'for'
for($i = $x = $z = 1; $i <= 10;$i++,$x+=2,$z=&$p){
   
    $p = $i + $x;
   
    print "\$i = $i , \$x = $x , \$z = $z <br />";
   
}

?>

lishevita at yahoo dot co (notcom) .uk (08-Sep-2006 08:33)

On the combination problem again...

 It seems to me like it would make more sense to go through systematically. That would take nested for loops, where each number was put through all of it's potentials sequentially.

The following would give you all of the potential combinations of a four-digit decimal combination, printed in a comma delimited format:

<?php
for($a=0;$a<10;$a++){
    for($b=0;$b<10;$b++){
          for($c=0;$c<10;$c++){
              for($d=0;$d<10;$d++){
                echo $a.$b.$c.$d.", ";
              }
           }
      }
}
?>

Of course, if you know that the numbers you had used were in a smaller subset, you could just plunk your possible numbers into arrays $a, $b, $c, and $d and then do nested foreach loops as above.

- Elizabeth

JustinB at harvest dot org (05-Aug-2005 12:23)

For those who are having issues with needing to evaluate multiple items in expression two, please note that it cannot be chained like expressions one and three can.  Although many have stated this fact, most have not stated that there is still a way to do this:

<?php
for($i = 0, $x = $nums['x_val'], $n = 15; ($i < 23 && $number != 24); $i++, $x + 5;) {
    // Do Something with All Those Fun Numbers
}
?>

user at host dot com (19-Apr-2004 11:53)

Also acceptable:

<?php
  for($letter = ord('a'); $letter <= ord('z'); $letter++)
   print chr($letter);
?>

bishop (17-Jul-2003 09:23)

If you're already using the fastest algorithms you can find (on the order of O(1), O(n), or O(n log n)), and you're still worried about loop speed, unroll your loops using e.g., Duff's Device:

<?php
$n = $ITERATIONS % 8;
while ($n--) $val++;
$n = (int)($ITERATIONS / 8);
while ($n--) {
    $val++;
    $val++;
    $val++;
    $val++;
    $val++;
    $val++;
    $val++;
    $val++;
}
?>

(This is a modified form of Duff's original device, because PHP doesn't understand the original's egregious syntax.)

That's algorithmically equivalent to the common form:

<?php
for ($i = 0; $i < $ITERATIONS; $i++) {
    $val++;
}
?>

$val++ can be whatever operation you need to perform ITERATIONS number of times.

On my box, with no users, average run time across 100 samples with ITERATIONS = 10000000 (10 million) is:
Duff version:       7.9857 s
Obvious version: 27.608 s

nzamani at cyberworldz dot de (18-Jun-2001 07:47)

The point about the speed in loops is, that the middle and the last expression are executed EVERY time it loops.
So you should try to take everything that doesn't change out of the loop.
Often you use a function to check the maximum of times it should loop. Like here:

<?php
for ($i = 0; $i <= somewhat_calcMax(); $i++) {
  somewhat_doSomethingWith($i);
}
?>

Faster would be:

<?php
$maxI = somewhat_calcMax();
for ($i = 0; $i <= $maxI; $i++) {
  somewhat_doSomethingWith($i);
}
?>

And here a little trick:

<?php
$maxI = somewhat_calcMax();
for ($i = 0; $i <= $maxI; somewhat_doSomethingWith($i++)) ;
?>

The $i gets changed after the copy for the function (post-increment).