Question

I had a head-scratcher yesterday.

Basically I had an associative array of some data with string keys(containing numeric values, but still, quoted strings) to go in a <select> box as options. I wanted to prepend a placeholder value to the select box’s options.

I attempted to use array_merge:

$placeholder = "Month";
$source = array
(
    '01' => '01 - January',
    '02' => '02 - February',
    '03' => '03 - March',
    '04' => '04 - April',
    '05' => '05 - May',
    '06' => '06 - June',
    '07' => '07 - July',
    '08' => '08 - August',
    '09' => '09 - September',
    '10' => '10 - October',
    '11' => '11 - November',
    '12' => '12 - December'
);

$source = array_merge(array('' => $placeholder), $source);

And I was seeing weird results — the resultant $source array was similar to the following:

(
    '' => 'Month',
    '01' => '01 - January',
    '02' => '02 - February',
    '03' => '03 - March',
    '04' => '04 - April',
    '05' => '05 - May',
    '06' => '06 - June',
    '07' => '07 - July',
    '08' => '08 - August',
    '09' => '09 - September',
    0 => '10 - October',
    1 => '11 - November',
    2 => '12 - December'
);

Note that it began using what appear to be auto-incrementing integer keys for October through December for some reason.

I can replace the array_merge call with this:

$source = array('' => $placeholder) + $source;

And the array doesn’t get rekeyed and everything is fine, but I don’t understand what’s happening under the scenes in the array_merge case.

Can anyone please explain what was happening to my array keys when I used array_merge? Thanks!

Answer 1

Problem doesn’t come from the array_merge() function.

If you print $source before merging, you’ll have this :

array(12) {
  ["01"]=>
  string(12) "01 - January"
  ["02"]=>
  string(13) "02 - February"
  // ...
}

Try to rename your first key '01' to '1', You will have the following return :

array(12) {
  [1]=>
  string(12) "01 - January"
  ["02"]=>
  string(13) "02 - February"
  // ...
}

PHP auto convert your key to integer. But I can’t say you why It does it.

EDIT :

I have found the answer on php.net (PHP.net) :

A key may be either an integer or a string. If a key is the standard
representation of an integer, it will be interpreted as such (i.e. “8”
will be interpreted as 8, while “08” will be interpreted as “08”).
Floats in key are truncated to integer. The indexed and associative
array types are the same type in PHP, which can both contain integer
and string indices.