lcwords.php

<?php

function lcwords($str) {
    $temp = '';

    $words = explode(' ', $str);

    foreach ($words as $word) {
        $word = ' ' . lcfirst($word);
        $temp .= $word;
    }

    return ltrim($temp);
}

$str = "Hello World Asd";
$str = lcwords($str);

print_r($str);

Implementation can be must cleaner and written as a one liner:

    function lcwords(string $str): string {
        return \implode(' ', \array_map('lcfirst', \explode(' ', $str)));
    }