5.7.1. The foreach Construct

$addresses = array('[email protected]', '[email protected]');
foreach ($addresses as $value) {
  echo "Processing $value\n";
}
Processing [email protected]
Processing [email protected]

PHP executes the body of the loop (the echo statement) once for each element of $addresses in turn, with $value set to the current element. Elements are processed by their internal order.

An alternative form of foreach gives you access to the current key:

$person = array('name' => 'Fred', 'age' => 35, 'wife' => 'Wilma');
foreach ($person as $k => $v) {
  echo "Fred's $k is $v\n";
}
Fred's name is Fred
Fred's age is 35
Fred's wife is Wilma

In this case, the key for each element is placed in $k and the corresponding value is placed in $v.

The foreach construct does not operate on the array itself, but rather on a copy of it. You can insert or delete elements in the body of a foreach loop, safe in the knowledge that the loop won't attempt to process the deleted or inserted elements.

5.7.2. The Iterator Functions

reset($addresses);
while (list($key, $value) = each($addresses)) {
  echo "$key is $value<BR>\n";
}
0 is [email protected]
1 is [email protected]

This approach does not make a copy of the array, as foreach does. This is useful for very large arrays when you want to conserve memory.

5.7.3. Using a for Loop

Here's how to print an array using for:

$addresses = array('[email protected]', '[email protected]');
for($i = 0; $i < count($array); $i++) {
  $value = $addresses[$i];
  echo "$value\n";
}
[email protected]
[email protected]

5.7.4. Calling a Function for Each Array Element

array_walk(array, function_name);

The function you define takes in two or, optionally, three arguments: the first is the element's value, the second is the element's key, and the third is a value supplied to array_walk( ) when it is called. For instance, here's another way to print table columns made of the values from an array:

function print_row($value, $key) {
  print("<tr><td>$value</td><td>$key</td></tr>\n");
}
$person = array('name' => 'Fred', 'age' => 35, 'wife' => 'Wilma');
array_walk($person, 'print_row');

A variation of this example specifies a background color using the optional third argument to array_walk( ). This parameter gives us the flexibility we need to print many tables, with many background colors:

function print_row($value, $key, $color) {
  print("<tr><td bgcolor=$color>$value</td><td bgcolor=$color>$key</td></tr>\n");
}
$person = array('name' => 'Fred', 'age' => 35, 'wife' => 'Wilma');
array_walk($person, 'print_row', 'blue');

The array_walk( ) function processes elements in their internal order.

5.7.5. Reducing an Array

$result = array_reduce(array, function_name [, default ]);

The function takes two arguments: the running total, and the current value being processed. It should return the new running total. For instance, to add up the squares of the values of an array, use:

function add_up ($running_total, $current_value) {
  $running_total += $current_value * $current_value;
  return $running_total;
}

$numbers = array(2, 3, 5, 7);
$total = array_reduce($numbers, 'add_up');
// $total is now 87

The array_reduce( ) line makes these function calls:

add_up(2,3)
add_up(13,5)
add_up(38,7)

The default argument, if provided, is a seed value. For instance, if we change the call to array_reduce( ) in the previous example to:

$total = array_reduce($numbers, 'add_up', 11);

The resulting function calls are:

add_up(11,2)
add_up(13,3)
add_up(16,5)
add_up(21,7)

If the array is empty, array_reduce( ) returns the default value. If no default value is given and the array is empty, array_reduce( ) returns NULL.

5.7.6. Searching for Values

if (in_array(to_find, array [, strict])) { ... }

If the optional third argument is true, the types of to_find and the value in the array must match. The default is to not check the types.

Here's a simple example:

$addresses = array('[email protected]', '[email protected]',
                   '[email protected]');
$got_spam = in_array('[email protected]', $addresses);   // $got_spam is true
$got_milk = in_array('[email protected]', $addresses);       // $got_milk is false

PHP automatically indexes the values in arrays, so in_array( ) is much faster than a loop that checks every value to find the one you want.

$person = array('name' => 'Fred', 'age' => 35, 'wife' => 'Wilma');
$k = array_search($person, 'Wilma');
echo("Fred's $k is Wilma\n");
Fred's wife is Wilma

The array_search( ) function also takes the optional third strict argument, which requires the types of the value being searched for and the value in the array to match.