Merge CSV

composer.json

{
    "require": {
        "league/csv": "^9.0",
        "symfony/finder": "^5.4"
    }
}

merge.php

<?php

require_once './vendor/autoload.php';

use League\Csv\Reader;
use League\Csv\Writer;
use Symfony\Component\Finder\Finder;

$finder = new Finder();
$finder->name('Pattern*')->in(__DIR__ . '/data');


$main = null;
$stat = [];
$rowCount = 0;
$mainHeader = null;

foreach ($finder as $file) {
    $absoluteFilePath = $file->getRealPath();
    $fileNameWithExtension = $file->getRelativePathname();

    $csv = Reader::createFromPath($absoluteFilePath, 'r');
    $csv->setDelimiter(',');
    $csv->setHeaderOffset(0);

    $json = $csv->jsonSerialize();
    $json = array_map(function($el) use ($fileNameWithExtension) {
        $el['__filename'] = $fileNameWithExtension;
        return $el;
    }, $json);
    $rowCount += count($json);
    $stat[$fileNameWithExtension] = count($json);
    if ($main == null) {
        $mainHeader = isset($json[0])? array_keys($json[0]):[];
        $main = $json;
    } else {
        array_push($main, ...$json);
    }
}

$records = [];
$records[] = [...$mainHeader];
$headerIndex = array_flip($records[0]);
$dummyRecord = [];
foreach ($mainHeader as $i => $header) {
    $dummyRecord[$i] = '';
}

$extraHeaders = [];
foreach ($main as $record) {
    $newRecord = [...$dummyRecord];
    foreach ($record as $key => $value) {
        $index = $headerIndex[$key] ?? null;
        if ($index !== null) {
            $newRecord[$index] = $value;
        } else {
            $extraHeaders[] = $key;
        }
    }
    $records[] = $newRecord;
}
var_dump(array_unique($extraHeaders));

$writer = Writer::createFromString('');
$writer->insertAll($records);
file_put_contents('./data/merged.csv', $writer->toString());