What is foreach in php process control

foreach (array_expression as $value)
    statementforeach (array_expression as $key => $value)    statement

<?php
$arr = array(1, 2, 3, 4);
foreach ($arr as &$value) {
    $value = $value * 2;
}
// $arr is now array(2, 4, 6, 8)
unset($value); // 最后取消掉引用
?>

<?php
foreach (array(1, 2, 3, 4) as &$value) {
    $value = $value * 2;
}

?>

<?php
$arr = array("one", "two", "three");
reset($arr);
while (list(, $value) = each($arr)) {
    echo "Value: $value<br>\n";
}

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

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

<?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";
}
?>

<?php
$array = [
    [1, 2],
    [3, 4],
];

foreach ($array as list($a, $b)) {
    // $a contains the first element of the nested array,
    // and $b contains the second element.
    echo "A: $a; B: $b\n";
}
?>

A: 1; B: 2
A: 3; B: 4

<?php
$array = [
    [1, 2],
    [3, 4],
];

foreach ($array as list($a)) {
    // Note that there is no $b here.
    echo "$a\n";
}
?>

1
3

<?php
$array = [
    [1, 2],
    [3, 4],
];

foreach ($array as list($a, $b, $c)) {
    echo "A: $a; B: $b; C: $c\n";
}
?>

Notice: Undefined offset: 2 in example.php on line 7
A: 1; B: 2; C: 

Notice: Undefined offset: 2 in example.php on line 7
A: 3; B: 4; C: