Script that splits one CSV into different CSV's by row count and stores results in `out` directory

split_csv_by_count.php

<?php

$filename = 'in.csv';
$outDir = 'out';

$in = fopen($filename, 'r');

$out = [];
$outName = pathinfo($filename, PATHINFO_FILENAME) . '_';
$extension = pathinfo($filename, PATHINFO_EXTENSION);

$first = true;
$fileNum = 1;
$rowNum = 1;
$rowsPerFile = 500;

if (file_exists($outDir)) {
    exec('rm -rf ' . escapeshellarg($outDir));
}
mkdir($outDir);

$out[$fileNum] = fopen(implode(DIRECTORY_SEPARATOR, [
    $outDir,
    $outName . $fileNum . '.' . $extension
]), 'w');
$headers = [];

while ($row = fgetcsv($in)) {
    if ($first) {
        $headers = $row;
        $first = false;

        fputcsv($out[$fileNum], $headers);
        continue;
    }

    if ($rowNum > $rowsPerFile) {
        fclose($out[$fileNum]);
        if (!isset($out[++$fileNum])) {
            $out[$fileNum] = fopen(implode(DIRECTORY_SEPARATOR, [
                $outDir,
                $outName . $fileNum . '.' . $extension
            ]), 'w');
            fputcsv($out[$fileNum], $headers);
        }

        $rowNum = 1;
    }

    fputcsv($out[$fileNum], $row);
    $rowNum++;
}

fclose($out[$fileNum]);
fclose($in);