Understanding differences in foreach() handling

In certain relatively obscure circumstances, the behavior of code inside a foreach() loop will vary between PHP 5 and PHP 7. First of all, there have been massive internal improvements, which means that in terms of sheer speed, processing inside the foreach() loop will be much faster running under PHP 7, compared with PHP 5. Problems that are noticed in PHP 5 include the use of current(), and unset() on the array inside the foreach() loop. Other problems have to do with passing values by reference while manipulating the array itself.

How to do it...

Consider the following block of code:

$a = [1, 2, 3];
foreach ($a as $v) {
  printf("%2d\n", $v);
  unset($a[1]);
}

In both PHP 5 and 7, the output would appear as follows:
```
 1
 2
 3
```

If you add an assignment before the loop, however, the behavior changes:

$a = [1, 2, 3];
$b = &$a;
foreach ($a as $v) {
  printf("%2d\n", $v);
  unset($a[1]);
}

Compare the output of PHP 5 and 7:

PHP 5

PHP 7

1

3

1

2

3
Working with functions that reference the internal array pointer also caused inconsistent behavior in PHP 5. Take the following code example:
```
$a = [1,2,3];
foreach($a as &$v) {
    printf("%2d - %2d\n", $v, current($a));
}
```
Tip
Every array has an internal pointer to its current element starting from 1, current() returns the current element in an array.
Notice that the output running in PHP 7 is normalized and consistent:

PHP 5

PHP 7

1 - 2

2 - 3

3 - 0

1 - 1

2 - 1

3 - 1
Adding a new element inside the foreach() loop, once the array iteration by reference is complete, is also problematic in PHP 5. This behavior has been made consistent in PHP 7. The following code example demonstrates this:
```
$a = [1];
foreach($a as &$v) {
    printf("%2d -\n", $v);
    $a[1]=2;
}
```
We will observe the following output:

PHP 5

PHP 7

1 -

1 -

2-
Another example of bad PHP 5 behavior addressed in PHP 7, during array iteration by reference, is the use of functions that modify the array, such as array_push(), array_pop(), array_shift(), and array_unshift().
Have a look at this example:
```
$a=[1,2,3,4];
foreach($a as &$v) {
    echo "$v\n";
    array_pop($a);
}
```
You will observe the following output:

PHP 5

PHP 7

1

2

1

1

1

2
Finally, we have a case where you are iterating through an array by reference, with a nested foreach() loop, which itself iterates on the same array by reference. In PHP 5 this construct simply did not work. In PHP 7 this has been fixed. The following block of code demonstrates this behavior:
```
$a = [0, 1, 2, 3];
foreach ($a as &$x) {
       foreach ($a as &$y) {
         echo "$x - $y\n";
         if ($x == 0 && $y == 1) {
           unset($a[1]);
           unset($a[2]);
         }
       }
}
```
And here is the output:

PHP 5

PHP 7

0 - 0

0 - 1

0 - 3

0 - 0

0 - 1

0 - 3

3 - 0

3 -3

PHP 5	PHP 7
1 3	1 2 3

PHP 5	PHP 7
1 - 2 2 - 3 3 - 0	1 - 1 2 - 1 3 - 1

PHP 5	PHP 7
1 -	1 - 2-

PHP 5	PHP 7
1 2 1 1	1 2

PHP 5	PHP 7
0 - 0 0 - 1 0 - 3	0 - 0 0 - 1 0 - 3 3 - 0 3 -3

How it works...

Add these code examples to a single PHP file, chap_02_foreach.php. Run the script under PHP 5 from the command line. The expected output is as follows:

Run the same script under PHP 7 and notice the difference:

Table of Contents for
PHP 7: Real World Application Development

Understanding differences in foreach() handling

How to do it...

Tip

How it works...

See also

Table of Contents for PHP 7: Real World Application Development

Understanding differences in foreach() handling

How to do it...

Tip

How it works...

See also

Table of Contents for
PHP 7: Real World Application Development