How to enforce a enclosure character using the League\Csv package

formatter.php

<?php

$addSequence = function (array $row) use ($sequence) {
    $res = [];
    foreach ($row as $value) {
        $res[] = $sequence.$value;
    }

    return $res;
};

$csv->addFormatter($addSequence);

register.php

<?php

RemoveSequence::registerStreamFilter();
$csv->appendStreamFilter(RemoveSequence::createFilterName($csv, $sequence));

RemoveSequence.php

<?php

namespace Example;

use League\Csv\AbstractCsv;
use php_user_filter;

class RemoveSequence extends php_user_filter
{
    const FILTER_NAME = 'removesequence.';

    const DELIMITER = '--';

    /**
     * The pattern to search for
     *
     * @var string
     */
    private $pattern;

    /**
     * The string to replace the pattern with
     *
     * @var string
     */
    private $replacement;

    /**
     * {@inheritdoc}
     */
    public function onCreate()
    {
        if (0 !== strpos($this->filtername, self::FILTER_NAME)) {
            return false;
        }

        return $this->isValidFiltername();
    }

    /**
     * Validate the filtername and set
     * the preg_replace pattern and replacement argument
     *
     * @return bool
     */
    private function isValidFiltername($str)
    {
        $settings = substr($this->filtername, strlen(self::FILTER_NAME));
        $res = explode(self::DELIMITER, $settings);
        $sequence = array_shift($res);
        $delimiter = array_shift($res);
        $enclosure = array_shift($res);
        if (is_null($sequence) || is_null($enclosure) || is_null($delimiter)) {
            return false;
        }

        $this->pattern = '/(^|'.preg_quote($delimiter).')'.preg_quote($enclosure).preg_quote($sequence).'/';
        $this->replacement = '$1'.$enclosure;
        return true;
    }

    /**
     * {@inheritdoc}
     */
    public function filter($in, $out, &$consumed, $closing)
    {
        while ($bucket = stream_bucket_make_writeable($in)) {
            $bucket->data = preg_replace($this->pattern, $this->replacement, $bucket->data);
            $consumed += $bucket->datalen;
            stream_bucket_append($out, $bucket);
        }

        return PSFS_PASS_ON;
    }

    /**
     * Register the generic class stream filter
     */
    public static function registerStreamFilter()
    {
        stream_filter_register(self::FILTER_NAME.'*', self::CLASS);
    }

    /**
     * Generate the specific stream filter for a given 
     * CSV class and a sequence
     *
     * @param AbstractCsv $csv      The object to which the filter will be attach
     * @param string      $sequence The sequence that will be removed from the CSV
     *
     * @throws InvalidArgumentException if the sequence contains invalid character
     *
     * @return string 
     */
    public static function createFilterName(AbstractCsv $csv, $sequence)
    {
        if (preg_match(',[\r\n\s],', $sequence)) {
            throw new InvalidArgumentException('The sequence contains invalid characters');
        }

        return self::FILTER_NAME.implode(self::DELIMITER, [$sequence, $csv->getDelimiter(), $csv->getEnclosure()]);
    }
}

script.php

<?php

use Example\RemoveSequence;
use League\Csv\Writer;

require 'vendor/autoload.php';

RemoveSequence::registerStreamFilter();

$sequence = "\t\x1f"; 

$addSequence = function (array $row) use ($sequence) {
    $res = [];
    foreach ($row as $value) {
        $res[] = $sequence.$value;
    }

    return $res;
};

$validateSequence = function (array $row) use ($sequence) {
    foreach ($row as $value) {
        if (0 !== mb_strpos($value, $sequence)) {
            return false;
        }
        $value = mb_substr($value, mb_strlen($sequence));
        if (false !== mb_strpos($value, $sequence)) {
            return false;
        }
    }

    return true;
};

$csv = Writer::createFromPath(__DIR__.'/test.csv', 'w');
$csv->addFormatter($addSequence);
$csv->addValidator($validateSequence, 'validate_sequence_presence');
$csv->appendStreamFilter(RemoveSequence::createFilterName($csv, $sequence));
$csv->insertOne(["\tfoo", 'bar', 'baz', "foo\tbar"]);
echo $csv->newReader('r'), PHP_EOL;
//you should display '"    foo","bar","baz","foo    bar"'

sequence.php

<?php

// The sequence prepended to each CSV cells to ensure SplFileObject::fputcsv adds
// the enclosure character on the cell
// https://github.com/php/php-src/blob/master/ext/standard/file.c#L1879
// '\t' seems to be the safest character to use.
// we add an extra character to make sure the sequence is unique for the CSV
// you can change the extra character depending on your CSV content
// The sequence should be as small as possible
// !WARNING: DO NOT USE THE NULL BYTE OR THE SPACE CHARACTER!
// Stream filter name can not contain null byte or space character and a RuntimeException will be thrown
$sequence = "\t\x1f";

validator.php

<?php

$validateSequence = function (array $row) use ($sequence) {
    foreach ($row as $value) {
        if (0 !== mb_strpos($value, $sequence)) {
            return false;
        }
        $value = mb_substr($value, mb_strlen($sequence));
        if (false !== mb_strpos($value, $sequence)) {
            return false;
        }
    }

    return true;
};

write.php

<?php

$csv->insertOne(["\tfoo", 'bar', 'baz', "foo\tbar"]);
echo $csv->newReader('r'), PHP_EOL;
//it should display '"    foo","bar","baz","foo    bar"'

writer-setup.php

<?php

$csv = Writer::createFromPath(__DIR__.'/test.csv', 'w');