// Created on savesnippets.com · https://savesnippets.com/O6LSpi02YkHS81
<?php
function normalizeHexColor(string $color): ?string {
    $hex = ltrim(trim($color), '#');
    if (!preg_match('/^(?:[0-9a-f]{3,4}|[0-9a-f]{6}|[0-9a-f]{8})$/i', $hex)) {
        return null;
    }
    $hex = strtolower($hex);
    // Expand 3/4-digit shorthand to 6/8-digit.
    if (in_array(strlen($hex), [3, 4], true)) {
        $expanded = '';
        foreach (str_split($hex) as $c) $expanded .= $c . $c;
        $hex = $expanded;
    }
    return '#' . $hex;
}

echo normalizeHexColor('#fff');        // #ffffff
echo normalizeHexColor('00aaffcc');    // #00aaffcc
var_dump(normalizeHexColor('xyz'));    // null