探讨PHP控制结构foreach的用法

foreach 语句用于循环遍历数组。每进行一次循环，当前数组元素的值就会被赋值给 value 变量（数组指针会逐一地移动） – 以此类推。

语法如下：

foreach (array as value)
{
    code to be executed;
}

Program List：循环输出数组的值

  
<?php
$arr = array("one", "two", "three");

foreach ($arr as $value)
{
	echo "Value: " . $value . "<br />";
}
?>

程序运行结果：

Value: one
Value: two
Value: three

Program List：循环输出数组的引用

从PHP 5开始，你可以通过在变量前面使用&操作符来修改数组的元素，这样的话分配的是引用，而不是值。

  
<?php
$arr = array(1, 2, 3, 4);
foreach ($arr as &$value) {
    $value = $value * 2;
	print $value.'<br />';
}
// $arr is now array(2, 4, 6, 8)
unset($value); // break the reference with the last element
?>

程序运行结果：

2
4
6
8

Program List：配合list()使用

    
<?php
$arr = array("Apple", "Banana", "Orange");
reset($arr);
while (list(, $value) = each($arr)) {
    echo "Fruit: $value<br />\n";
}

foreach ($arr as $value) {
    echo "Fruit: $value<br />\n";
}
?>

程序运行结果：

Fruit: Apple
Fruit: Banana
Fruit: Orange
Fruit: Apple
Fruit: Banana
Fruit: Orange

Program List：输出键与值

    
<?php
$arr = array("Apple", "Banana", "Orange");
reset($arr);
while (list($key, $value) = each($arr)) {
    echo "Key: $key; Value: $value<br />\n";
}

foreach ($arr as $key => $value) {
    echo "Key: $key; Value: $value<br />\n";
}
?>

程序运行结果：

Key: 0; Value: Apple
Key: 1; Value: Banana
Key: 2; Value: Orange
Key: 0; Value: Apple
Key: 1; Value: Banana
Key: 2; Value: Orange

Program List：更多foreach的使用例子

<?php
/* foreach example 1: value only */

$a = array(1, 2, 3, 17);

foreach ($a as $v) {
    echo "Current value of \$a: $v.\n";
}

/* foreach example 2: value (with its manual access notation printed for illustration) */

$a = array(1, 2, 3, 17);

$i = 0; /* for illustrative purposes only */

foreach ($a as $v) {
    echo "\$a[$i] => $v.\n";
    $i++;
}

/* foreach example 3: key and value */

$a = array(
    "one" => 1,
    "two" => 2,
    "three" => 3,
    "seventeen" => 17
);

foreach ($a as $k => $v) {
    echo "\$a[$k] => $v.\n";
}

/* foreach example 4: multi-dimensional arrays */
$a = array();
$a[0][0] = "a";
$a[0][1] = "b";
$a[1][0] = "y";
$a[1][1] = "z";

foreach ($a as $v1) {
    foreach ($v1 as $v2) {
        echo "$v2\n";
    }
}

/* foreach example 5: dynamic arrays */

foreach (array(1, 2, 3, 4, 5) as $v) {
    echo "$v\n";
}
?>