深入理解SET NAMES和mysql(i)_set_charset的区别

//mysql-5.1.30-SRC/libmysql/client.c, line 3166:
int STDCALL mysql_set_character_set(MYSQL *mysql, const char *cs_name)
{
  struct charset_info_st *cs;
  const char *save_csdir= charsets_dir;
 
  if (mysql->options.charset_dir)
    charsets_dir= mysql->options.charset_dir;
 
  if (strlen(cs_name) < MY_CS_NAME_SIZE &&
     (cs= get_charset_by_csname(cs_name, MY_CS_PRIMARY, MYF(0))))
  {
    char buff[MY_CS_NAME_SIZE + 10];
    charsets_dir= save_csdir;
    /* Skip execution of "SET NAMES" for pre-4.1 servers */
    if (mysql_get_server_version(mysql) < 40100)
      return 0;
    sprintf(buff, "SET NAMES %s", cs_name);
    if (!mysql_real_query(mysql, buff, strlen(buff)))
    {
      mysql->charset= cs;
    }
  }
  //以下省略

sprintf(buff, "SET NAMES %s", cs_name);
if (!mysql_real_query(mysql, buff, strlen(buff)))
{
  mysql->charset= cs;
}

<?php
    $db = mysql_connect('localhost:3737', 'root' ,'123456');
    mysql_select_db("test");
    $a = "\x91\x5c";//"慭"的gbk编码, 低字节为5c, 也就是ascii中的"\"
 
    var_dump(addslashes($a));
    var_dump(mysql_real_escape_string($a, $db));
 
    mysql_query("set names gbk");
    var_dump(mysql_real_escape_string($a, $db));
 
    mysql_set_charset("gbk");
    var_dump(mysql_real_escape_string($a, $db));
?>

$ php -f 5c.php
string(3) "慭\"
string(3) "慭\"
string(3) "慭\"
string(2) "慭"