Obscure arbitrary security-sensitive string in PHP

obscure.php

<?php

    /**
     * Obscures sensitive string (like a security token),
     * but preserves minimal information about letters, digits, spaces and unknown characters.
     *
     * @param string $string
     *
     * @return string
     */
    protected function obscure(string $string)
    {
        // Input:  '_ ?$$EFdasdasda das56 14a6512341 2341 )_% UIML: '
        // Output: '?_???aaaaaaaaaa_aaa00_00a0000000_0000_???_aaaa?_'

        $obscured = preg_replace('/[^a-z\s\d]/i', '?', $string);
        $obscured = preg_replace('/[a-z]/i', 'a', $obscured);
        $obscured = preg_replace('/\d/', '0', $obscured);
        $obscured = preg_replace('/\s/', '_', $obscured);

        return $obscured;
    }