Question

I have a question about PHP and the use of pointers and variables.

The following code produces something I wouldn’t have expected:

<?php
$numbers = array('zero', 'one', 'two', 'three');

foreach($numbers as &$number)
{
  $number = strtoupper($number);
}

print_r($numbers);

$texts = array();
foreach($numbers as $number)
{
  $texts[] = $number;
}

print_r($texts);
?>

The output is the following

Array
(
    [0] => ZERO
    [1] => ONE
    [2] => TWO
    [3] => THREE
)
Array
(
    [0] => ZERO
    [1] => ONE
    [2] => TWO
    [3] => TWO
)

Notice the ‘TWO’ appearing twice in the second array.

It seems that there is a conflict between the two foreach loops, each declaring a $number variable (once by reference and the second by value).

But why ? And why does it affect only the last element in the second foreach ?

Answer 1

The key point is that PHP does not have pointers. It has references, which is a similar but different concept, and there are some subtle differences.

If you use var_dump() instead of print_r(), it’s easier to spot:

$collection = array(
    'First',
    'Second',
    'Third',
);

foreach($collection as &$item){
    echo $item . PHP_EOL;
}

var_dump($collection);

foreach($collection as $item){
    var_dump($collection);
    echo $item . PHP_EOL;
}

… prints:

First
Second
Third
array(3) {
  [0]=>
  string(5) "First"
  [1]=>
  string(6) "Second"
  [2]=>
  &string(5) "Third"
}
array(3) {
  [0]=>
  string(5) "First"
  [1]=>
  string(6) "Second"
  [2]=>
  &string(5) "First"
}
First
array(3) {
  [0]=>
  string(5) "First"
  [1]=>
  string(6) "Second"
  [2]=>
  &string(6) "Second"
}
Second
array(3) {
  [0]=>
  string(5) "First"
  [1]=>
  string(6) "Second"
  [2]=>
  &string(6) "Second"
}
Second

Please note the & symbol that’s left in the last array item.

To sum up, whenever you use references in a loop, it’s good practice to remove them at the end:

<?php

$collection = array(
    'First',
    'Second',
    'Third',
);

foreach($collection as &$item){
    echo $item . PHP_EOL;
}
unset($item);

var_dump($collection);

foreach($collection as $item){
    var_dump($collection);
    echo $item . PHP_EOL;
}
unset($item);

… prints the expected result every time.