function random_password($length, $characters='abcdefgh1234567890'){
 
    if ($characters == ''){ return ''; }
    $chars_length = strlen($characters)-1;
 
    mt_srand((double)microtime()*1000000);
 
    $pwd = '';
    while(strlen($pwd) < $length){
        $rand_char = mt_rand(0, $chars_length);
        $pwd .= $characters[$rand_char];
    }
 
    return $pwd;
 
}

//使用范例
random_password(5,'abcd1234') => returns something like: '2b4d3'