Simple RGB->Hex and Hex->RGB converter.

rgbhex.php

<?php
/**
 * HEX to RGB and RGB to HEX converter
 * Thanks to hafees@msn.com on php.net for the base
 * http://www.php.net/manual/en/function.hexdec.php#99478
 */

/**
 * Convert an RGB array to HEX
 *
 * @param array $colors array('r' => 255, 'g' => 125, 'b' => 0)
 * @return string 
 */
function rgb_to_hex($colors = array()) {
        /* Won't accept black, so we make an exception (thanks Arantor) */
        if ($colors['r'] === '0' && $colors['g'] === '0' && $colors['b'] === '0')
                return '000000';
        /* If it's an empty array, or missing a color, it's invalid! */
        if (empty($colors['r']) && empty($colors['g']) & empty($colors['b']))
                return false;

        return strtoupper(dechex($colors['r']).dechex($colors['g']).dechex($colors['b']));
}

/**
 * Convert a HEX color string to an RGB array
 *
 * @param string $hex HEX string to convert
 * @param bool $returnstring Return as string or array
 * @return array
 */
function hex_to_rgb($hex, $returnstring = false) {
        $hex = preg_replace('~[^0-9A-Fa-f]~', '', $hex);
        $rgb = array();

        if (strlen($hex) == 6) {
                $color = hexdec($hex);
                $rgb['r'] = 0xFF & ($color >> 0x10);
                $rgb['g'] = 0xFF & ($color >> 0x8);
                $rgb['b'] = 0xFF & $color;
        } elseif (strlen($hex) == 3) {
                $rgb['r'] = hexdec(str_repeat(substr($hex, 0, 1), 2));
                $rgb['g'] = hexdec(str_repeat(substr($hex, 1, 1), 2));
                $rgb['b'] = hexdec(str_repeat(substr($hex, 2, 1), 2));
        } else
                return false;

        return $returnstring == true ? implode(',', $rgb) : $rgb;
}

echo rgb_to_hex(array('r' => '0', 'g' => '0', 'b' => '0'));
echo '<br />';
echo hex_to_rgb('1234ef', true);