[nerbert]

You have to split the string up into an array. Here's what I got working:

function rainbow($str)
{
	$array = str_split($str);    
        $i = $b = 0;
        $str = '';
        $colours = array('d31539', 'ff7e00', 'ffc20e', '90d125', '187acb', '6f3198', 'ab1d8e'); 
        while( $i < count($array) ) 
        { 
                if( $b > ( count( $colours ) - 1 ) ) $b = 0; 
                 
                $str .= '<span style="color: #' . $colours[ $b ] . '">' . $array[$i] . '</span>'; 
                 
                ++$b; 
                ++$i; 
        }
        return $str;
}