Twig filter for `merge_concat` between arrays, making easier to add classes and csv lists as attributes.

MergeConcatTwigExtension.php

<?php

/**
 * Copyright © 2015 Berny Cantos <[email protected]>
 *
 * This work is free. You can redistribute it and/or modify it under the
 * terms of the Do What The Fuck You Want To Public License Version 2 as
 * published by Sam Hocevar.
 *
 * See http://www.wtfpl.net/about file for more details.
 */

class MergeConcatTwigExtension extends \Twig_Extension
{
    /**
     * Returns a list of filters to add to the existing list.
     *
     * @return array An array of filters
     */
    public function getFilters()
    {
        return array(
            new \Twig_SimpleFilter('merge_concat', function ($source, $strings, $trim = ' ') {
                return $this->mergeConcat($source, $strings, $trim);
            }),
        );
    }

    /**
     * Merge string values in an array concatenating them with an optional separator
     *
     * @param array  $source    Array to filter
     * @param array  $strings   Array of strings to concat into $source
     * @param string $separator Optional character separator (may be empty)
     *
     * @return array
     */
    protected function mergeConcat(array $source, array $strings, $separator)
    {
        if (!is_array($source)) {
            throw new \UnexpectedValueException(
                'Filtered value should be an array'
            );
        }

        if (!is_array($strings)) {
            throw new \UnexpectedValueException(
                'Merging value should be an array'
            );
        }

        $merged = array();
        foreach ($strings as $key => $value) {
            if (array_key_exists($key, $source)) {
                $oldValue = $source[$key];

                if (is_array($oldValue)) {
                    $value = $this->mergeConcat($source[$key], $value, $separator);

                } else if (is_string($oldValue)) {
                    $value = $source[$key].$separator.$value;

                } else {
                    throw new \UnexpectedValueException(
                        'Only merge strings or arrays'
                    );
                }
            }

            $merged[$key] = is_string($value) && !empty($separator)
                ? trim($value, $separator)
                : $value;
        }

        return array_merge($source, $merged);
    }

    /**
     * Returns the name of the extension.
     *
     * @return string The extension name
     */
    public function getName()
    {
        return 'merge_concat';
    }
}