initials.php

<?php

/**
 * @param string $string
 * @param array $avoid (['de', 'da', 'a', 'e', 'o'])
 * @return string
 */
function initials(string $string, array $avoid = null): string
{
    $words = preg_split("/\s+/", $string);
    $restrict = $avoid ? $avoid : ['de', 'da', 'a', 'e', 'o'];
    $input = array_diff($words, $restrict);
    $function = function ($carry, $item) {
        $carry[] = ((string)$item){0};
        return $carry;
    };
    return implode('', array_reduce($input, $function, []))
}

// para funcionar no php 5.6

function initials($string, $avoid = null)
{
    $words = preg_split("/\s+/", $string);
    $restrict = $avoid ? $avoid : ['da', 'de', 'do', 'a', 'e', 'o'];
    $input = array_diff($words, $restrict);

    $function = function ($carry, $item) {
        $carry[] = (string) $item[0];
        return $carry;
    };

    return implode('', array_reduce($input, $function, []));
}