merge_csvs.php

<?php
$row = 1;
$fileNo = 1;
$outfp = fopen(__DIR__."/out.csv", "w+");

$files = glob("*.csv");
$originalHeaders = [];
foreach ($files as $file)
{
  $row = 0;
  if (($handle = fopen($file, "r")) !== FALSE) {
      $headers = [];
      while (($data = fgetcsv($handle, 1000, ",")) !== FALSE) {
          $row++;
          if ($row == 1) {
            $headers = $data;
            if ($fileNo == 1) {
              $originalHeaders = $headers;
              fputcsv($outfp, $originalHeaders);
            }
            continue;
          }

          $num = count($data);
          $line = array_combine($headers, $data);
          $sortedLine = sortToOriginalHeaders($originalHeaders, $line);
          fputcsv($outfp, $sortedLine);
      }
      fclose($handle);
  }
  $fileNo++;
}

function sortToOriginalHeaders($originalHeaders, $line) {
  var_dump($line, $originalHeaders);

  $output = [];
  foreach ($originalHeaders as $key => $value) {
    $output[$key] = $line[$value];
  }

  return $output;
}