How can I update an array value and put it into a mysql database using php?

Question

Okay so a while ago I asked this question

How can I grab and store multiple $_POST Variables in a for each loop?

And the response was perfect, and it worked for inserting the values into the database just fine. However, I now am revisiting this project and I want to UPDATE only one field in the database, specifically the post test field for that particular user, using the same or similar method. How would this be possible?

Currently I have something along the lines of this.

$SGUID = the Unique User ID
$SGpost = the field i want to update
$SGQID = question ID

Obviously, this does not work.. and my code is extremely sloppy as I tried to replace and edit this along the way. Any ideas?

foreach ($_POST as $p => $v)
{

 if (preg_match("/^Q\d+$/",$p)) {   //$_POST variables with Q1, Q2, Q3, etc etc

$query_3 = mysql_query("UPDATE SGresults SET SGpost = '$p', SGpostCheck = 1 WHERE SGUID = '$SGUID' AND SGQID = '$p'") or die('Error: '.mysql_error ());
    $SGQID++;

}
}
}   

When I echo out $p I get the following Q11 Q21 Q31 Q44 Q54 Q63 Q73 Q83 Q93 Q103 Q111 Q122 Q131 Q142 Q153 Q163 Q17

Which is what I want, except without the Q, and just the second value. So for example if it was 'Q163' I just want to insert the '3' part into SGpost WHERE the database is 'Q16'.

Thanks again again! Any help would be greatly appreciated!

EDIT:: Looked over my code again and changed some values around...The following DOES work, so I think I will stick with it. Unless bad code? Thanks again for your suggestions.

$SGQID= 1;

foreach ($_POST as $i => $v)
  {
    if (preg_match("/^Q\d+$/",$i)) {    //$_POST variables with Q1, Q2, Q3, etc etc
// Here we insert value $v to database
    if ($SGtestType == "Post-test") {

    $query_post = mysql_query("UPDATE SGresults SET SGpost = '$v', SGpostcheck ='1' WHERE SGQID = '$SGQID' AND SGUID = '$SGUID'  ") or die('Error 109: '.mysql_error ());
    $SGQID++;           
    // ^^ increments the question value everytime an insert is made
    //last 2 fields is check for existing value.. or if not exists.. 1 if it does.
    }

}

}
}

Answer 1

Assuming you trust the post data to not be an xss attack:

    foreach ($_POST as $p => $v)
    {

     if (preg_match("/^Q(\d+)$/",$p, $matches)) {   //$_POST variables with Q1, Q2, Q3, etc etc

$id = (int)$matches[1];

    $query_3 = mysql_query("UPDATE SGresults SET SGpost = '$id', SGpostCheck = 1 WHERE SGUID = '$SGUID' AND SGQID = '$id'") or die('Error: '.mysql_error ());
        $SGQID++;

    }
    }
    }