Preserving numerical array keys while merging

20 May 2013

Here’s one I recently learned. Given two arrays:

$old = array(
    103 => 'Value for 103',
    53 => 'Value for 53'
);

$new = array(
    53 => 'New value to replace old 53',
    9376 => 'Value for 9376'
);

array_merge will not preserve numeric keys, meaning that subsequently added items will be added as new values even if you don’t intend them to be. To preserve the keys use the array union operator:

var_dump(array_merge($old, $new));

array (size=4)
    0 => string 'Value for 103' (length=13)
    1 => string 'Value for 53' (length=12)
    2 => string 'New value to replace old 53' (length=27)
    3 => string 'Value for 9376' (length=14)

var_dump($new + $old);

array (size=3)

    53 => string 'New value to replace old 53' (length=27)
    9376 => string 'Value for 9376' (length=14)
    103 => string 'Value for 103' (length=13)

Common knowledge, perhaps, but this was a new one for me :)