The obvious way to do this is with a subselect, which mySQL doesn’t have. So how does one do it?

At first I thought that it might be impossible, but I have figured out the answer:

SELECT f.forum_id,
       max(c.created_on) as last_date,
       c2.created_on as created_on,
       c2.text
FROM   forum f
       LEFT JOIN comment c ON (c.forum_id = f.forum_id)
       LEFT JOIN comment c2 ON (f.forum_id = c2.forum_id) 
GROUP BY f.forum_id, c2.comment_id
HAVING created_on = last_date OR last_date IS NULL;

See, it’s pretty clever actually, though it leans on the “HAVING” option, which is sort of crappy. The trick is to join the comment table with the forum table twice. One of those joins gets grouped by forum id so that you can use the max function on it. The other doesn’t get grouped (by adding its primary key to the group by clause) so that you can still pull data out of it. Then you use the having clause to find only the row that has the max date.

I am also accepting values with a max value of NULL so that if a forum has no comments, it still comes back in the results.

Hope this helps someone, somewhere.
Happy Hacking

mysql_query(): 14 is not a valid MySQL-Link resource in <bla bla bal> on line 47

A few other people seem to have gotten this error, but no one has posted a solution. (though one guy oh-so-annoyingly posted “I figured it out, so never mind” … Grrr. I mean, if you’re going to post a question, the answer should be in that thread if you ever figure it out….

So as soon as I figure out the answer I’m going to post it here.

I’m back with the solution:

I got clued onto it from this page: http://bytes.com/forum/thread638479.html . The error is coming because mysql_close was being called by the destructor, and because I was in safe mode the same MySQL resource was being used for each instance. What threw me even more though was that the destructor was being called at all, because I thought I only _had_ one instance. Turns out that there is a spot in my code where I (accidentaly) passed my DB object by value rather than refrence. This made a new copy of the object, which ran mysql_connect again, because it was in safe mode it returened the _same_ refrence. Then the object got unloaded, the destructor ran and closed the refrence, even though there was another instance of the object out there still using the same refrence.

Icky!

This will be the first post in a series where I go through this code and explain it, and some of the cooler calls that it is using, as well as some advanced php and programming concepts. But for now I’m just going to give a one paragraph overview of what the code does and get it into the post. There is one other file which I will also post and explain at some point.

So read On to take a look at the nitty-gritty, and get a feel for what it does in total

I call this program the introspective debugger. The basic Idea is that you include this file as the first line of any script you are running, and then when you visit the page, rather than getting a normal page, you end up with an interactive debugger. This is done with frames. In one frame you can look at all of the variables, or at the source code with your current line highlighted or the stack trace, In another you can look at the output so far. Sadly at this time you can’t change variables on the fly. You can then tell the program to run again until it hits the next break-point, or until it ends.

Here is the full source code for introspective debugger:

<?php
$idbg_var_exclude_list = array('GLOBALS', '_FILES', '_COOKIE', '_POST', '_GET', 'idbg_var_exclude_list' , 'idbg_stop_flag', 'idbg_var_spy', 'idbg_backtrace');
$old_error_level = error_reporting(0);
if(array_key_exists("idbg_id",$_GET)){
  $idbg_id = $_GET["idbg_id"];
} else {
  $idbg_id = 1025 + rand(5000,8000); // get a random high port
}
$callBackName= htmlspecialchars_decode($_SERVER['SCRIPT_NAME']."?idbg_id=$idbg_id&idbg_mode=");
$callBackWvars= htmlspecialchars_decode($_SERVER['REQUEST_URI'].(strpos($_SERVER['REQUEST_URI'],"?")?"&":"?")."idbg_id=$idbg_id&idbg_mode=");
if(!array_key_exists("idbg_mode",$_GET)){
  /* Display Frame Mode */
  echo("<html>
 <frameset rows='65%,35%'>
   <frame src='$callBackName"."cnc' name='infoframe'>
   <frame src='$callBackWvars"."run' name='scriptframe'>
</frameset>
</html>");
  exit();

} else if (array_key_exists("idbg_mode",$_GET) and ($_GET["idbg_mode"] == "cnc" or $_GET["idbg_mode"] == "fstep")){
  if(array_key_exists("idbg_act",$_GET)){ idbg_socket_client($_GET["idbg_act"]);}
  $responce = unserialize(idbg_socket_client("cnc"));
  if($_GET["idbg_mode"] == "fstep"){ header( "Location: ".$callBackName."file&idbg_open_line=".$responce['trace'][0]['line']. "&idbg_open=".$responce['trace'][0]['file']."#anchor");}
  echo ("<h1> I am Command & Control DIV $idbg_id </h1>");
  echo("[<a href='".$callBackName."cnc' target='infoframe'>CNC</a>] ");
  echo("[<a href='".$callBackName."cnc&idbg_act=step' target='infoframe'>STEP</a>] ");
  echo("[<a href='".$callBackName."cnc&idbg_act=cont' target='infoframe'>CONTINUE</a>]<br/><br/><br/>");
  foreach($responce["trace"] as $key => $val){
    echo("<a href='".$callBackName."file&idbg_open_line=".$val['line'].
	 "&idbg_open=".$val['file']."#anchor' target='infoframe'>".strrchr($val['file'],"/")."</a>");
    echo(" Line:". $val['line'] . " Function:" . $responce["trace"][$key+1]["function"]. "
  <a href='#' onClick="document.getElementById('v_$key').style.display='block'">+</a>
          <div id='v_$key' style='display:none'><pre>");
    print_r($responce["vars"][$responce["trace"][$key+1]["function"]]);
    echo("</pre></div><hr/>");
  }
  exit();

} else if (array_key_exists("idbg_mode",$_GET) && $_GET["idbg_mode"] == "file"){
  echo("[<a href='".$callBackName."cnc' target='infoframe'>CNC</a>] ");
  echo("[<a href='".$callBackName."fstep&idbg_act=step' target='infoframe'>STEP</a>] ");
  echo("[<a href='".$callBackName."fstep&idbg_act=cont' target='infoframe'>CONTINUE</a>]<br/><br/><br/>
        <div style='background-color:#CCC'>");
   $fp = fopen($_GET["idbg_open"],"r");
   $current_line = 0;
   while(!feof($fp)){
    $current_line++;
    if($current_line < $_GET["idbg_open_line"]){
      $top .= (fgets($fp));
    }else if($current_line == $_GET["idbg_open_line"]){
      $keyline.= (fgets($fp));
    } else {
      $bottom .= (fgets($fp));
    }
   }

   echo(str_replace("a7R4449p","<a name='anchor'/>
        <div style='background-color:#BBF;border: thin solid #000'>",str_replace("b7R4449p","</div>",
	highlight_string($top . "a7R4449p". $keyline ."b7R4449p" . $bottom, TRUE))));
   echo("</div>");
  fclose($fp);
  exit();

} else if (array_key_exists("idbg_mode",$_GET) && $_GET["idbg_mode"] == "run"){
  echo ("<h1> I am running DIV $idbg_id </h1>");
  register_tick_function("idbg_tick_function");
}

function idbg_socket_client($msg){
  global $idbg_id;
  //echo ("=>".$msg."<BR/>");flush();
  do {
    usleep(rand(50000,150000));
    $conn = stream_socket_client("tcp://".$_SERVER["REMOTE_ADDR"].":$idbg_id", $errno, $errstr);
    //echo($errstr . "<= ");flush();
  } while($errno == 111);

  if (!$conn) { echo "Debugger couldn't visit socket: $errstr ($errno)<br />\n"; } else {
    fwrite($conn, $msg);
    while (!feof($conn)) {
      $responce .= fgets($conn, 1024);
      //echo ("->".$responce);flush();
    }
    fclose($conn);
    return($responce);
  }
}

function idbg_tick_function(){
  global $idbg_stop_flag, $idbg_id, $idbg_var_spy, $idbg_last_line, $idbg_backtrace;
  $idbg_backtrace = debug_backtrace();
  //  echo ($idbg_last_line. " ?= " .$idbg_backtrace[0]['line'] ." in ".$idbg_backtrace[0]['file']."<Br/>" );
  if($idbg_stop_flag and $idbg_last_line != $idbg_backtrace[0]['line'] and !strpos($idbg_backtrace[0]['file'], 'STOP.php')){
    //echo("</hr> idbg_tick_function START <br/>");
    $socket = stream_socket_server("tcp://".$_SERVER["REMOTE_ADDR"].":$idbg_id", $errno, $errstr);
    if (!$socket) { echo "<h1>Debugger couldn't open socket: $errstr ($errno)<h1/>\n";} else {
      while ($conn = stream_socket_accept($socket)) {
	$msgIn = fread($conn, 1024);
	echo($msgIn);
	flush();
	if ($msgIn == 'cnc'){
	  fwrite($conn, serialize(array("trace"=>$idbg_backtrace,"vars" => $idbg_var_spy)));
	} else if ($msgIn == 'step'){
	  fclose($conn);
	  fclose($socket);
	  $idbg_last_line = $idbg_backtrace[0]['line'];
	  return(TRUE);
	} else if ($msgIn == 'cont'){
	  //	  fwrite($conn, "ACK");
	  fclose($conn);
	  fclose($socket);
	  $idbg_last_line = 0;
	  $idbg_stop_flag = FALSE;
	  return(TRUE);
	} else {
	  fwrite($conn, " This is a SERVER Responce \n");
	}
	fclose($conn);
      }
      fclose($socket);
    }
  }
}//else just keep going
error_reporting($old_error_level);

?>