在一些小型的应用中，完全没有必要使用大型数据库软件。自己做一个SQL解释器就能用数据库的方式来管理了。
这个解释器，能解释常用的SQL命令。你可以自行添加其他功能。

<?php
class DB_text {
  var $conn;
  var $classname = "db_text";
  var $database;
  function on_create() {
  }
  function connect($database_name) {
    $this->database = $database_name;
    if(! file_exists($database_name)) {
      $this->conn = array();
      $this->_close();
    }
    $fp = fopen($this->database,"r");
    $this->conn = unserialize(fread($fp,filesize($this->database)));
    fclose($fp);
  }
  function &query($query) {
    if(eregi("select ",$query)) return $this->_select($query);
    if(eregi("insert ",$query)) return $this->_insert($query);
    if(eregi("delete ",$query)) return $this->_delete($query);
    if(eregi("update ",$query)) return $this->_update($query);
    return array();
  }
  function fetch_row(&$result) {
    if(list($key,$value) = each($result))
      return $value;
    return false;
  }
  function num_rows($result) {
    return count($result);
  }

  /**
   * query的辅助函数
   */
  function _select($query) {
    if(eregi("(order by (.+))",$query,$regs)) {
      $order = $regs[2];
      $query = eregi_replace($regs[1],"",$query);
    }
    if(eregi("(group by (.+))",$query,$regs)) {
      $group = $regs[2];
      $query = eregi_replace($regs[1],"",$query);
    }
    eregi("select .* from ([0-9a-z_]+) *(where +(.+))?",$query,$regs);
    if($regs[3] != "") {
      $keys = $this->_where($regs[3],"\$this->conn[$regs[1]]");
      while(list($key,$value) = each($keys)) {
        $rs[] = $this->conn[$regs[1]][$value];
      }
    }else {
      $rs = $this->conn[$regs[1]];
    }
    if($order) {
      sscanf($order,"%s %s",$key,$type);
      if(empty($type)) $type = "asc";
        $this->_sort($rs,$key,$type);
    }
    return $rs;
  }
  function _insert($query) {
    eregi("insert +into +([0-9a-z_]+) *(.+) *values? *(.+)",$query,$regs);
    eval("\$key=array$regs[2];");
    eval("\$value=array$regs[3];");
    for($i=0;$i<count($key);$i++)
      $rs[$key[$i]] = $value[$i];
    $this->conn[$regs[1]][] = $rs;
    $this->_close();
  }
  function _update($query) {
    eregi("update +([0-9a-z_]+) +set *(,?.*=.*)+( +where +(.+))",$query,$regs);
    $regs[2] = eregi_replace(",","=",$regs[2]);
    $v = split("=",$regs[2]);
    $keys = $this->_where($regs[4],"\$this->conn[$regs[1]]");
    while(list($key,$value) = each($keys)) {
      for($i=0;$i<count($v);$i+=2)
        $this->conn[$regs[1]][$value][$v[$i]] = eregi_replace("","",$v[$i+1]);
    }
    $this->_close();
  }
  function _delete($query) {
    eregi("delete +from +([0-9a-z_]+) *(where +(.+))?",$query,$regs);
    $keys = $this->_where($regs[3],"\$this->conn[$regs[1]]");
    while(list($key,$value) = each($keys)) {
      unset($this->conn[$regs[1]][$value]);
    }
    reset($this->conn[$regs[1]]);
    while(list($key,$value) = each($this->conn[$regs[1]])) {
      $ch[] = $value;
    }
    $this->conn[$regs[1]] = $ch;
    $this->_close();
  }
  function _where($search,$table) {
    $search = eregi_replace("\("," ( ",$search);
    $search = eregi_replace("\)"," ) ",$search);
    $search = eregi_replace("\+"," + ",$search);
    $search = eregi_replace("\*"," * ",$search);
    while(eregi("[^ ]([*/><!=-])",$search,$regs)) {
      $search = eregi_replace($regs[1]," $regs[1] ",$search);
    }
    while(eregi("([><!] +=)",$search,$regs)) {
      $search = eregi_replace($regs[1],eregi_replace(" ","",$regs[1]),$search);
    }
    $search = eregi_replace("  "," ",trim($search));
    $search = eregi_replace(" and "," && ",$search);
    $search = eregi_replace(" or "," || ",$search);
    $search = eregi_replace(" = "," == ",$search);
    $ar = split(" ",$search);
    eval("\$t=$table;");

    for($i=0;$i<count($ar);$i++) {
      if(isset($t[0][$ar[$i]]))
        $ar[$i] = "\$value[".$ar][$i]."]";
    }
    $expr = "\$expl=(".join(" ",$ar).");";
   
    while(list($key,$value) = each($t)) {
      eval($expr);
      if($expl)
        $keys[] = $key;
    }
    return $keys;
  }
  function _sort(&$ar,$key=0,$mode="desc") {
    global $cmp_key;
    $cmp_key = $key;
    if($mode == "asc")
      usort($ar,_cmp_asc);
    else
      usort($ar,_cmp_desc);
  }
  function _close() {
    $fp = fopen($this->database,"w");
    fwrite($fp,serialize($this->conn));
    fclose($fp);
  }
}

/** 排序键
*/
$cmp_key = "";

/** 排序用工作函数(降序 由usort()调用)
*/
function _cmp_desc($a,$b) {
  global $cmp_key;
  if ($a[$cmp_key] == $b[$cmp_key]) return 0;
  return ($a[$cmp_key] > $b[$cmp_key]) ? -1 : 1;
}

/** 排序用工作函数(升序 由usort()调用)
*/
function _cmp_asc($a,$b) {
  global $cmp_key;
  if ($a[$cmp_key] == $b[$cmp_key]) return 0;
  return ($a[$cmp_key] > $b[$cmp_key]) ? 1 : -1;
}
?>

测试例：
<pre>
<?php
//require_once "db_text.php";

$conn = new DB_text;
$conn->connect("text1.txt");

$conn->query("insert into manage (id,title) values (10,abcd)");
$conn->query("insert into manage (id,title) values (2,43d)");
$conn->query("insert into manage (id,title) values (20,tuu)");
$conn->query("update manage set id=101,test=a where id=10");
//$conn->query("delete from manage where id=10");
//$conn->query("delete from manage where id=10 or table=code");


//$rt = $conn->query("select * from manage where id=101 or table=code group by 1 order by 1 asc");
$rt = $conn->query("select * from manage group by 1 order by id desc");

print_r($rt);

?>
</pre>