Why is there a "&" in ($arr as &$value)?

$arr = array(1, 2, 3, 4);
foreach ($arr as &$value) {
    $开发者_运维百科value = $value * 2;
}

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

Does $key mean index of an array?

&$variable_name is PHP's way of doing pass by reference:

See: http://php.net/manual/en/language.references.pass.php

For more of an explaination of what references are see: http://www.php.net/manual/en/language.references.whatare.php

    $arr = array(1, 2, 3, 4);

    foreach ($arr as $value)
       $value = $value * 100;
    echo implode(' ', $arr); // 1 2 3 4

    foreach ($arr as &$value)
        $value = $value * 100;
    echo implode(' ', $arr); // 100 200 300 400

got it?

You have two different questions. To answer your first about the ampersand (&):

Whenever you see the & passed in front of a variable, it means that you will be working with the actual variable and not a copy of it. With your example, the $value would typically be a copy of the actual $value, so any changes made to that variable would not affect the array. But when you put the & in front of it, you are working with actual array data.

To answer your second question, yes, the $key is an index of the array, whether it's numerical or associative.

The & tells PHP to make $value a reference to the actual array item, instead of a copy of the array item. Without the &, your code will not actually modify the values within the array.

The & allows you to change the values in $value. You can experiment by trying it without. That's called passing by reference.

The example here isn't so great for explaining $key because you show a regular array rather than an associative array. Here's a better example to illustrate $key.

$a = array(
  "red" => 1,
  "green" => 2,
  "blue" => 3,
  "white" => 17 /* btw, I patent the white pixel! */
);

foreach ($a as $key => $value) {
  echo "key $key, val $value";
}