Array_merge versus + [duplicate]

Question

This question already has an answer here:

• What's the difference between array_merge and array + array? 9 answers

When I use array_merge() with associative arrays I get what I want, but when I use them with numerical key arrays the keys get changed.

With + the keys are preserved but it doesn't work with associative arrays.

I don't understand how this works, can anybody explain it to me?

Answer 1

Because both arrays are numerically-indexed, only the values in the first array will be used.

The + operator returns the right-hand array appended to the left-hand array; for keys that exist in both arrays, the elements from the left-hand array will be used, and the matching elements from the right-hand array will be ignored.

http://php.net/manual/en/language.operators.array.php

array_merge() has slightly different behavior:

If the input arrays have the same string keys, then the later value for that key will overwrite the previous one. If, however, the arrays contain numeric keys, the later value will not overwrite the original value, but will be appended. Values in the input array with numeric keys will be renumbered with incrementing keys starting from zero in the result array.

http://php.net/manual/en/function.array-merge.php

Answer 2

These two operation are totally different.

array plus

1. Array plus operation treats all array as assoc array.
2. When key conflict during plus, left(previous) value will be kept
3. null + array() will raise fatal error

array_merge()

1. array_merge() works different with index-array and assoc-array.
2. If both parameters are index-array, array_merge() concat index-array values.
3. If not, the index-array will to convert to values array, and then convert to assoc array.
4. Now it got two assoc array and merge them together, when key conflict, right(last) value will be kept.
5. array_merge(null, array()) returns array() and got a warning said, parameter #1 is not an array.

I post the code below to make things clear.

function array_plus($a, $b){
    $results = array();
    foreach($a as $k=>$v) if(!isset($results[$k]))$results[$k] = $v;
    foreach($b as $k=>$v) if(!isset($results[$k]))$results[$k] = $v;
    return $results;
}

//----------------------------------------------------------------

function is_index($a){
    $keys = array_keys($a);
    foreach($keys as $key) {
        $i = intval($key);
        if("$key"!="$i") return false;
    }
    return true;
}

function array_merge($a, $b){
    if(is_index($a)) $a = array_values($a);
    if(is_index($b)) $b = array_values($b);
    $results = array();
    if(is_index($a) and is_index($b)){
        foreach($a as $v) $results[] = $v;
        foreach($b as $v) $results[] = $v;
    }
    else{
        foreach($a as $k=>$v) $results[$k] = $v;
        foreach($b as $k=>$v) $results[$k] = $v;
    }
    return $results;
}