Hey, I just impressed myself a little. I thought this code was going to take a little longer than it did, but I got it essentially on the first try, and quickly.
<?php
function merge(&$a, &$b){
$keys = array_keys($a);
foreach($keys as $key){
if(isset($b[$key])){
if(is_array($a[$key]) and is_array($b[$key])){
merge($a[$key],$b[$key]);
}else{
$a[$key] = $b[$key];
}
}
}
$keys = array_keys($b);
foreach($keys as $key){
if(!isset($a[$key])){
$a[$key] = $b[$key];
}
}
}
?>
I actually got it on the second try, but only because I forgot that second part that copies the values for the keys that didn't exist in the first array from the second (duh).
Anyway, I figured it was neat, so I blogged it. 
I was going to go into a longer post about why none of array_merge, array_merge_recursive, and the normal array '+' operator that I didn't even know about did what I wanted, but I still have to study for my probability and statistics exam tomorrow.
Update: I knew I should have waited to post this. There's a bug if the two arrays don't have the same structure. In other words, if one has an array for a given key where the other has a scalar, it won't work. But I think it's only if the first array has a scalar and the second one has an array, and not the other way around. I'll post a corrected version here when I get it right
Update: Ok, fixed. Neglected to check that both $a[$key] and $b[$key] were arrays. So now it works exactly like I expected it to. However, because of how my CMS works, the way I planned it isn't actually the way I need it to work So I'll have to deal with that later...
Update: Over in the HiveMinds forum there's a discussion of this code:
Now, I took a couple passes over the code, and it looks like to me that if a value for the same key exists in both arrays, the value from array b will overwrite the value in array a. What am I missing here? How is this truly a merge? I see how the second block adds to a things that don't exist in b, but the first code block (other than the section that calls merge recursively if both keys are really arrays) seems to overwrite the slot in a with the value in b. Maybe thats just what he wants it to do. Am I missing something here? I'm sure its right in front of me and I'm just not seeing it.
That's what I intended. I mentioned above that I didn't have time to explain why PHP's built in array_merge and array_merge_recursive functions, and the built-in array '+' operator weren't what I needed. But what the poster describes is exactly what I intended. Notice, however, that this code does in fact behave similarly to the array_merge function and the built-in array '+' operator (except maybe for numeric indices -- I have to make sure), except that those aren't recursive.
The reason I needed this type of merge function is because, in my CMS, "more specific" settings overwrite "less specific" settings as I parse through the configuration file. Unfortunately, this code doesn't do what my CMS needs when the arrays have numeric indices.
Update: check out the new version.
Sunday, March 30, 2003
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Thanks for the explanation and the link to Hiveminds.
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