array_merge is Useful - But With a Caveat

Oct 7, 2008 php

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So, the other day, I saw a horrible thing. I saw two PHP associative arrays that needed to be combined into one, and the worst example of NOT using PHP’s built in functions to combine them. They weren’t using array_merge - instead they were looping through each value.

That’s what I thought until I did some testing. There is a legitimate difference in the looping method vs the array_merge method. This could be by design in your application, so don’t get over-eager optimizing. Let’s take a look:

Example Arrays

$ar1 = array('a'=>'ay', 'b'=>'bee', 'c'=>'see');
$ar2 = array('d'=>'dee', 'e'=>'ee', 'f'=>'ef');

Well, first off, let’s try my way - with array_merge:

$ar2 = array_merge($ar1, $ar2);
var_dump($ar2);

Output:

array(6) { ["a"]=>  string(2) "ay" ["b"]=>  string(3) "bee"
["c"]=>  string(3) "see" ["d"]=>  string(3) "dee"
["e"]=>  string(2) "ee" ["f"]=>  string(2) "ef" }

Ok - decent. Now let’s try it their way:

foreach ($ar1 as $k=>$v) {
  $ar2[$k]=$v;
}
var_dump($ar2);

Output:

array(6) { ["d"]=>  string(3) "dee" ["e"]=>  string(2) "ee"
["f"]=>  string(2) "ef" ["a"]=>  string(2) "ay"
["b"]=>  string(3) "bee" ["c"]=>  string(3) "see" }

The array is in a different order.

Special Thanks to Sjan and James for commenting on my original version of this story and explaining that I was totally running in circles!