PHP merge arrays based on matching value of a given index

Simplest solution to make it completely dynamic apart from ID, do like below:

$arr2 = array_column($array2, "ID");

$finalArray = array();
foreach($array1 as $arr){
    $key = array_search($arr['ID'], $arr2);
    if($key ===false){
        $key = array_search(0, $arr2);
    }
    unset($array2[$key]['ID']);
    $finalArray[] =     array_merge($arr,$array2[$key]);
}

print_r($finalArray);

Output:- https://3v4l.org/1sDJs

First use the ID as keys:

$arr1 = array_column($arr1, null, "ID");
$arr2 = array_column($arr2, null, "ID");

Then sort the key (to solve non-same original indexes:

ksort($arr1);
ksort($arr2);

Then use array_map as:

$res = array_map('array_merge', $arr1, $arr2);

Use array_values at the end to ignore the ID keys.

And if you have another array that include the ID field you can have the same logic on them - notice array_map can get more then 2 input array...

Reference: array_column, array_map, array_merge, ksort

Live example: 3v4l

Edited:

Used template to fix issue where element are missing:

$keys = array_merge(array_keys($a[0]), array_keys($b[0]));
$template = array_combine($keys, array_fill(0, count($keys), null)); // create array of null for all keys
$a = array_column($a, null, "ID");
$b = array_column($b, null, "ID");
ksort($a);
ksort($b);


$res = array_map(function ($e1, $e2) use ($template){
    if ($e1 && $e2) return array_merge($e1, $e2); // if exist merge
    $e = $e1 ? $e1 : $e2; //get the exist element
    return array_replace ($template, $e); // add it
}, $a, $b);

Notice this solution will be valid to only 2 array - it can be generalize but I leave that to you

Live example: 3v4l

I tweak your code to do what you need, here is the complete code that worked for me:

<?php

$arr=Array(
     Array("ID" => 101, "Code" => 1075, "Date" => "2012-03-03 17:13:12.433"),
    Array("ID" => 103, "Code" => 175, "Date" => "2013-03-03 17:13:12.433"),
    Array("ID" => 109, "Code" => 178, "Date" => "2014-03-03 17:13:12.433")
);

$arr4 = Array(
    Array("ID" => 101, "Amount" => 1234),
    Array("ID" => 103, "Amount" => 1342),
    Array("ID" => 0, "Amount" => 0)
);

$combined = array();

foreach ($arr as $i => $arrs) {
    $combined['ID'][$i] = $arrs['ID'];
    $combined['Code'][$i] = $arrs['Code'];
    $combined['Date'][$i] = $arrs['Date'];

    foreach ($arr4 as $arr2) {
        if ($arr2['ID'] == $arrs['ID']) {
            $combined['Amount'][$i] = $arr2['Amount'];
            break;
        }
    }

    if (!isset($combined['Amount'][$i])) {
        $combined['Amount'][] = 0;
    }

}

echo print_r($combined);

?>

If you need further manipulation for different reasons, i suggest you read about array_keys and array_values methods, which are very helpful.

$firstArray = [
    [
        'ID' => 101,
        'Code' => 1075,
        'Date' => '2012-03-03 17:13:12.433',
    ],
    [
        'ID' => 103,
        'Code' => 175,
        'Date' => '2012-09-05 20:30:02.217',
    ],
    [
        'ID' => 109,
        'Code' => 178,
        'Date' => '2012-07-05 20:30:02.217',
    ],
];

$secondArray = [
    [
        'ID' => 101,
        'Amount' => 1234,
    ],
    [
        'ID' => 103,
        'Amount' => 1342,
    ],
    [
        'ID' => 0,
        'Amount' => 0,
    ],
];

class DTO implements \Serializable
{
    /** @var int */
    private $id;

    /** @var int */
    private $code;

    /** @var string */
    private $date;

    /** @var int */
    private $amount = 0;

    /**
     * {@inheritDoc}
     */
    public function serialize()
    {
        return [
            'ID' => $this->id,
            'Code' => $this->code,
            'Date' => $this->date,
            'Amount' => $this->amount,
        ];
    }

    /**
     * {@inheritDoc}
     *
     * @param array $serialized
     */
    public function unserialize($serialized): self
    {
        $this->id = $serialized['ID'];
        $this->code = $serialized['Code'];
        $this->date = $serialized['Date'];
        $this->amount = $serialized['Amount'] ?? 0;

        return $this;
    }
}

$thirdArray = [];
foreach ($secondArray as $item) {
    $id = $item['ID'];
    unset($item['ID']);
    $thirdArray[$id] = $item;
}

$thirdArrayKeys = array_keys($thirdArray);
foreach ($firstArray as &$item) {
    $id = $item['ID'];
    $dto = new DTO();
    $dto->unserialize($item);
    if (in_array($id, $thirdArrayKeys)) {
        $dto = $dto->unserialize(array_merge($item, $thirdArray[$id]));
    }

    $item = $dto->serialize();
}

http://sandbox.onlinephpfunctions.com/code/ee72a1c89782f4eef25a1143816f237ca6af129c