This function is for parsing legacy color values. It is based on the WHATWG specs.

parseColorString.php

<?php
function parseColorString($str, $format = 'hex') {
    $str = strtolower(trim($str));

    // prevent errors for an empty string and ignore 'transparent'
    if ($str == '' || $str == 'transparent') {
        return false;
    }

    // replace nonvalid hex characters with 0
    $str = preg_replace('/[^0-9a-f]/u', 0, $str);

    // zero-pad the string to the next multiple of 3,
    // so it can be split into 3 parts of the same size
    $nextMultiple = ceil(strlen($str) / 3) * 3;
    $str = str_pad($str, $nextMultiple, 0);

    $colorChannels = str_split($str, strlen($str) / 3);

    // truncate from left to (rightmost) 8 chars
    foreach ($colorChannels as &$channel) {
        $channel = substr($channel, -8);
    }

    // remove leading zeros equally on all three color channels
    while ($colorChannels[0][0] . $colorChannels[1][0] . $colorChannels[2][0] == '000') {
        $colorChannels[0] = substr($colorChannels[0], 1);
        $colorChannels[1] = substr($colorChannels[1], 1);
        $colorChannels[2] = substr($colorChannels[2], 1);
    }

    // cut or prepend with zeros until 2 chars long
    foreach ($colorChannels as &$channel) {
        $channel = str_pad($channel, 2, 0, STR_PAD_LEFT);
        $channel = substr($channel, 0, 2);
    }

    switch ($format) {
        case '':
            return $colorChannels;
            break;
        case 'rgb':
            return vsprintf('rgb(%d, %d, %d)', array_map('hexdec', $colorChannels));
            break;
        case 'hex':
        default:
            return vsprintf('#%s%s%s', $colorChannels);
    }
}