PHP: Return all unique characters present in an array of strings.

char_list.php

<?php

/**
 * Return all unique characters present in an array of strings.
 * It ignores spaces due to join().
 * e.g.:
 * ['abc', 'g', 'def', 'abcd'];
 * > 'abcdefg'
 *
 * @param array $array
 *
 * @return string
 */
function charList(array $array)
{
  $letters = array_unique(preg_split('//u', join('', $array)));
  sort($letters);

  return join('', $letters);
}