Tóm Tắt
foreach
(PHP 4, PHP 5, PHP 7, PHP 8)
The foreach construct provides an easy way to
iterate over arrays. foreach works only on arrays
and objects, and will issue an error when you try to use it on a variable
with a different data type or an uninitialized variable. There are two
syntaxes:
foreach (iterable_expression as $value)
statement
foreach (iterable_expression as $key => $value)
statement
The first form traverses the iterable given by
iterable_expression. On each iteration, the value of
the current element is assigned to $value.
The second form will additionally assign the current element’s key to
the $key variable on each iteration.
Note that foreach does not modify the internal array
pointer, which is used by functions such as current()
and key().
It is possible to
customize object iteration.
In order to be able to directly modify array elements within the loop precede
$value with &. In that case the value will be assigned by
reference.
<?php
$arr
= array(
1
,
2
,
3
,
4
);
foreach (
$arr
as &
$value
) {
$value
=
$value
*
2
;
}
// $arr is now array(2, 4, 6, 8)
unset(
$value
);
// break the reference with the last element
?>
Warning
Reference of a $value and the last array element
remain even after the foreach loop. It is recommended
to destroy it by unset().
Otherwise you will experience the following behavior:
<?php
$arr
= array(
1
,
2
,
3
,
4
);
foreach (
$arr
as &
$value
) {
$value
=
$value
*
2
;
}
// $arr is now array(2, 4, 6, 8)
// without an unset($value), $value is still a reference to the last item: $arr[3]
foreach (
$arr
as
$key
=>
$value
) {
// $arr[3] will be updated with each value from $arr...
echo
"
{
$key
}
=>
{
$value
}
"
;
print_r
(
$arr
);
}
// ...until ultimately the second-to-last value is copied onto the last value
// output:
// 0 => 2 Array ( [0] => 2, [1] => 4, [2] => 6, [3] => 2 )
// 1 => 4 Array ( [0] => 2, [1] => 4, [2] => 6, [3] => 4 )
// 2 => 6 Array ( [0] => 2, [1] => 4, [2] => 6, [3] => 6 )
// 3 => 6 Array ( [0] => 2, [1] => 4, [2] => 6, [3] => 6 )
?>
It is possible to iterate a constant array’s value by reference:
<?php
foreach (array(
1
,
2
,
3
,
4
) as &
$value
) {
$value
=
$value
*
2
;
}
?>
Note:
foreachdoes not support the ability to
suppress error messages using
@.
Some more examples to demonstrate usage:
<?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"
;
}
?>
Unpacking nested arrays with list()
(PHP 5 >= 5.5.0, PHP 7, PHP 8)
It is possible to iterate over an array of arrays and unpack the
nested array into loop variables by providing a list()
as the value.
For example:
<?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"
;
}
?>
The above example will output:
A: 1; B: 2 A: 3; B: 4
You can provide fewer elements in the list() than there
are in the nested array, in which case the leftover array values will be
ignored:
<?php
$array
= [
[
1
,
2
],
[
3
,
4
],
];
foreach (
$array
as list(
$a
)) {
// Note that there is no $b here.
echo
"
$a
\n"
;
}
?>
The above example will output:
1 3
A notice will be generated if there aren’t enough array elements to fill
the list():
<?php
$array
= [
[
1
,
2
],
[
3
,
4
],
];
foreach (
$array
as list(
$a
,
$b
,
$c
)) {
echo
"A:
$a
; B:
$b
; C:
$c
\n"
;
}
?>
The above example will output:
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:
