Foreach

In PHP we have loop syntax that is similar to other languages. We use foreach to enumerate the elements of a collection in order.

And with for, and while, we have more traditional loops—we can use these loops to iterate over indexes of a collection. Do-while can be used, but it is slightly more difficult.

Example

Consider this example PHP program. We create an array of 3 strings at its start—the array is created without using the array() keyword.

Version 1 We use foreach. It is important to note that each individual element is the second part, after the ampersand in the foreach.

Version 2 We use for, starting at index 0 and proceeding until the count() of the array is reached.

Version 3 We use while, starting the variable "i" at 0, and incrementing it until we reach the count().

Version 4 Here we use do-while. This is similar to while, but it does not check the count before entering the loop.

// An array of 3 strings.
$elements = ["one", "two", "three [END]"];

// Version 1: use foreach on array.
foreach ($elements as &$element) {
    var_dump($element);
}

// Version 2: use for-loop over array.
for ($i = 0; $i < count($elements); $i++) {
    var_dump($elements[$i]);
}

// Version 3: use while-loop over array.
$i = 0;
while ($i < count($elements)) {
    var_dump($elements[$i]);
    $i++;
}

// Version 4: use do-while loop over array.
$i = 0;
do {
    var_dump($elements[$i]);
    $i++;
} while ($i < count($elements));
string(3) "one"
string(3) "two"
string(11) "three [END]"
string(3) "one"
string(3) "two"
string(11) "three [END]"
string(3) "one"
string(3) "two"
string(11) "three [END]"
string(3) "one"
string(3) "two"
string(11) "three [END]"

Foreach, mutable

We can mutate elements in an array that we access through a foreach loop, but we must use the ampersand character. This creates a mutable reference.

Version 1 We increment each element in the array by using an ampersand in the foreach. Our array ends up with 2, 3, and 4.

Version 2 We try to increment each value in the array, but it only increments a copy of each element. The original array remains unchanged.

// Version 1: modify the array with & in foreach.
$elements = array(1, 2, 3);
foreach ($elements as &$element) {
    $element++;
}
var_dump($elements);

// Version 2: cannot modify array without the & character.
$elements = array(1, 2, 3);
foreach ($elements as $element) {
    $element++;
}
var_dump($elements);array(3) {
  [0]=>
  int(2)
  [1]=>
  int(3)
  [2]=>
  &int(4)
}
array(3) {
  [0]=>
  int(1)
  [1]=>
  int(2)
  [2]=>
  int(3)
}

Generator

It is possible to call a special generator function with a foreach-loop. In a generator, the yield statement returns repeatedly, "yielding" many values to the loop.

function first_ten() {
    // This generator yields the first 10 integers.
    for ($i = 0; $i < 10; $i++) {
        yield $i;
    }
}

// Call generator in foreach.
foreach (first_ten() as $value) {
    var_dump($value);
}int(0)
int(1)
int(2)
int(3)
int(4)
int(5)
int(6)
int(7)
int(8)
int(9)

Clearest syntax

The foreach-loop is the clearest way to write a simple loop over the elements of an array. But if we have complex requirements, or need the index, for may be better.

And While and do-while are usually poor choices for looping over arrays. But for when no array is involved, they can be useful.

For writing PHP programs, loops are almost always needed. And choosing the clearest loop is an essential skill—foreach, with its "as" syntax, is an ideal choice.