PHP array, foreach loop with references and no block-scope – a killer recipe

Today morning I was browsing through my collection ridiculous php bugs ( I have 20+ such code snippet now) and my eyes got stuck to this problem. I cannot remember from where I got this one but surely it’s a jewel in my collection.

Let give a look to the original code
<?php

   $arr = array('A', 'B', 'C', 'D', 'E');

   foreach ($arr as &$val) {}

   foreach ($arr as $val) {}

   var_dump($arr);

?>

The problem is that the var_dump gives a result

array(5) {

  [0]=>

  &string(1) "A"

  [1]=>

  &string(1) "B"

  [2]=>

  &string(1) "C"

  [3]=>

  &string(1) "D"

  [4]=>

  &string(1) "D"

}

Shocked! Aaha… who screwed up your array? Apparently it’s your references
and variable scopes. You get it right but you will be biased if you don’t
criticize the lack of block-scope in php. 

In the first foreach block we are assigning reference of the each element
to $val. Thus at the end of the first foreach loop $val actually a reference
to the final index of array $arr.

Now as there is no concept of block-scope in php, the $val will retain the
reference to the final index $arr at the start of the second loop (pathetic!).

 We can simply represent the situation as bellow

<?php

   $arr = array('A', 'B', 'C', 'D', 'E');

   $val = &$arr[4] ;

   foreach ($arr as $val) {}

   var_dump($arr);

?>

Now doing a var_dump of $arr inside the second foreach loop gives us the
following result.

array (

  0 => 'A',

  1 => 'B',

  2 => 'C',

  3 => 'D',

  4 => 'A',

)

array (

  0 => 'A',

  1 => 'B',

  2 => 'C',

  3 => 'D',

  4 => 'B',

)

array (

  0 => 'A',

  1 => 'B',

  2 => 'C',

  3 => 'D',

  4 => 'C',

)

array (

  0 => 'A',

  1 => 'B',

  2 => 'C',

  3 => 'D',

  4 => 'D',

)

array (

  0 => 'A',

  1 => 'B',

  2 => 'C',

  3 => 'D',

  4 => 'D',

) 

So it is clear now the second foreach loop actually assigns value of
each index one by one to the index referred by $val (final index).

 Now try out this solution 

<?php

   $arr = array('A', 'B', 'C', 'D', 'E');

   foreach ($arr as &$val) {}

   unset($val);

   foreach ($arr as $val) {}

   var_dump($arr);

?>

IT WORKS :)

(c) Sourav Ray