<?php

    function hex2rgba($hex, $a = 0)
    {
        $hex = ltrim($hex, '#');

        if (strlen($hex) == 3) {
            $hex = $this->completeHex($hex);
        }

        $this->checkHexFormat($hex);

        $hexArray = str_split($hex,2); //for get r-g-b values

        $hexArray = array_map(function ($item) {
            return hexdec($item);
        }, $hexArray);

        $hexArray['alpha'] = $a;

        return "rgb($hexArray[0], $hexArray[1], $hexArray[2], $hexArray[alpha])";
    }

    function completeHex($hex)
    {
        return $hex[0].$hex[0].$hex[1].$hex[1].$hex[2].$hex[2];
    }

    function checkHexFormat($hex)
    {
        if (strlen($hex) !== 6) {
            throw new Exception('hex code can be 6 digit');
        }

        if (ctype_xdigit($hex) == false)
        {
            throw new Exception('unknown format');
        }
    }

?>