Transwiki:PHP Programming/The foreach Loop

The code

foreach ($array as $someVar) {
  echo ($someVar . "<br />");
}

or

foreach ($array as $key => $someVar) {
  echo ($key." holds the value ".$someVar."<br />");
}

Analysis

The foreach loop is a special form of the standard for loop. The example above will print all the values of $array. The foreach structure is a convenient way to loop through an array.

Simple foreach statement

Foreach loops are useful when dealing with an array indexed with arbitrary keys (e.g. non-numeric ones):

$array = array(
  "1st" => "My House",
  "2nd" => "My Car",
  "3rd" => "My Lab"
);

To use the classical for structure, you'd have to write:

// get all the array keys
$arrayKeys = array_keys($array);
// get amount of values from array
$count = count($array);
// loop through the keys
for ($i = 0; $i < $count; $i++) {
  // get each array value using its key
  echo $array[($arrayKeys[$i])] . "<br />";
}

Basically, an array value can be accessed only from its key: to make sure you get all the values, you first have to make a list of all the existings keys, then grab all the corresponding values. The access to first array value, the previous example does the following steps:

$firstKey = $arrayKeys[0];       // which is '1st'
$firstValue = $array[$firstKey]; // which is 'My House' ($array('1st'))

The foreach structure does all the groundwork for you:

foreach ($array as $someVar) {
  echo $someVar . "<br />";
}

Note how the latter example is easier to read (and write). Both will output:

My House
My Car
My Lab

foreach with key values

If you need to use the array's keys in the body of your loop, just add the variable as in the following statement. The phrase '$myKey => $value' makes the value of the key accessible:

foreach ($array as $myKey => $value) {
  // use $myKey
}

Note that this is very useful when constructing a dropdown list in HTML. You can use a foreach-loop to have the $myKey variable inserted into the value="..." part and the $value as the actual text.

Note that, if you change the assigned variable inside the foreach loop, the change will not be reflected to the array. Therefore, if you need to change elements of the array you need to change them by using the array key. Example:

$array = array(
  "1st" => "My House",
  "2nd" => "My Car",
  "3rd" => "My Lab"
);

foreach ($array as $i => $someVar) {
  // OK
  if($someVar == 'My Lab') {
    $array[$i] = 'My Laboratory';
  }

  // doesn't update the array
  $someVar = 'Foo';
}