config.yml

FormCaptureAdmin:
  extensions:
    - FormCaptureAdminExtension

FormCaptureAdminExtension.php

<?php

class FormCaptureAdminExtension extends Extension
{
	public function updateEditForm(Form $form)
	{
		$gridField = $form->Fields()->dataFieldByName('CapturedFormSubmission');
		$gridField->getConfig()->removeComponentsByType('GridFieldExportButton');
		$gridField->getConfig()->removeComponentsByType('GridFieldPrintButton');
		$gridField->getConfig()->addComponent(new FormCaptureGridFieldExportButton('buttons-before-left'));
	}
}

FormCaptureGridFieldExportButton.php

<?php

use League\Csv\Writer;
use League\Flysystem\Filesystem;
use League\Flysystem\ZipArchive\ZipArchiveAdapter;

class FormCaptureGridFieldExportButton extends GridFieldExportButton
{
	/**
	 * {@inheritdoc}
 	 */
	public function handleExport($gridField, $request = null)
	{
		$now = date("d-m-Y-H-i");
		$fileName = "export-$now.zip";

		// We write multiple CSVs to a ZIP to be downloaded
		$filesystem = new Filesystem(new ZipArchiveAdapter(TEMP_FOLDER . '/' . $fileName));

		// Get a list of form types - each will have its own CSV file
		$types = CapturedFormSubmission::get()->alterDataQuery(function ($dataQuery) {
			$dataQuery->groupBy('Type');
		})->column('Type');

		foreach ($types as $type) {
			$csv = Writer::createFromString('');
			$submissions = CapturedFormSubmission::get()->filter(['Type' => $type]);
			$fields = $submissions->relation('CapturedFields');

			// Fetch header row data and push it to the CSV
			$headers = $this->getHeadersForFields($fields);
			$csv->insertOne($headers);

			// We have no guarantees about field order matching the header order, so we have to extract
			// all the fields into an array first and then check it against the headers later. Alternative
			// would be to $submission->CapturedFields()->filter(['Type' => $header])... but that would
			// result in thousands of extra queries and be incredibly slow
			$rawData = $this->getDataFromFields($fields);

			// Now we've got an array of _all_ the fields we received in an unknown order...
			foreach ($rawData as $submissionID => $submissionData) {
				$data = [];

				// ...we can loop over it in the same order as the headers
				foreach ($headers as $header) {
					$data[] = isset($submissionData[$header]) ? $submissionData[$header] : null;
				}

				// Push the data to the CSV
				$csv->insertOne($data);
			}

			// Add the CSV file to the ZIP
			$filesystem->put($type . '.csv', $csv->__toString());
			gc_collect_cycles();
		}

		// This has to be called manually as PHP's ZipArchive normally closes on __destruct()
		$filesystem->getAdapter()->getArchive()->close();

		// Grab the contents... using streams would be a nice enhancement
		$contents = file_get_contents(TEMP_FOLDER . '/' . $fileName);
		unlink(TEMP_FOLDER . '/' . $fileName);

		if (!$contents) {
			return null;
		}

		return SS_HTTPRequest::send_file($contents, $fileName, 'application/zip');
	}

	/**
	 * @param DataList $fields
	 * @return array
	 */
	protected function getHeadersForFields(DataList $fields)
	{
		// We don't know the headers, so we query them here
		$headers = $fields->alterDataQuery(function ($dataQuery) {
			$dataQuery->groupBy('Name'); // Quicker than fetching them all and doing array_unique()
		})->column('Name');

		// Try to put the headers in a sane order
		$preferred = [
			'PersonTitle', 'Title', 'FirstName', 'Surname', 'EmailAddress', 'TelephoneNumber',
			'MobileNumber', 'Address1', 'Address2', 'Town', 'County', 'Postcode', 'Country'
		];
		usort($headers, function ($a, $b) use ($preferred) {
			$aKey = array_search($a, $preferred);
			$bKey = array_search($b, $preferred);

			if ($aKey === false) return 1;
			if ($bKey === false) return -1;

			return ($aKey > $bKey) ? 1 : -1;
		});

		// Push "Created", as that's a useful field
		array_unshift($headers, 'Created');

		return $headers;
	}

	/**
	 * Build an array of data from submitted fields, grouped by submission ID
	 *
	 * @param DataList $fields
	 * @return array
	 */
	protected function getDataFromFields(DataList $fields)
	{
		// Use raw database values rather than casted DB fields - much, much faster
		$query = $fields->dataQuery();

		$data = [];
		foreach ($query->execute() as $capturedField) {
			$submissionID = $capturedField['FormSubmissionID'];
			if (!isset($data[$submissionID])) {
				// Bonus! Created date always comes first
				$data[$submissionID] = [
					'Created' => $capturedField['Created']
				];
			}

			$data[$submissionID][$capturedField['Name']] = $capturedField['Value'];
		}

		return $data;
	}
}