PHP中的嵌套MySQL查询

Question

I came across the following code while constructing a php file:

  $query="UPDATE EXPERTS SET ...";  
  $result=$db->query($query);         
      if ($result) 
      {     
        $query="UPDATE USERS SET ...";  
        $result=$db->query($query);     
        if ($result) 
        {       
          ...
        }
      }

My question is if the above nested query is acceptable since we use the same variable ($result). Of course, it works, however I am not sure if is there any case of creating unexpected results...

Thank you

Answer 1

You can use Data Flow Anomalies for this:

Required terminology:

• d($var) - Value assigned to $var or initialized.
• r($var) - Value of $var read.
• u($var) - Value of $var is undefined.

We wish to avoid the following patterns:

• ur - A undefined value is read.
• du - A variable which once had a value becomes undefined before it's read.
• dd - A variable receives a new value before it's earlier value is properly used; this is probably what you're afraid of.

Now let's take a look:

$query="UPDATE EXPERTS SET ...";            
$result=$db->query($query);                        d($result)
if ($result)                                       r($result)
{     
    $query="UPDATE USERS SET ...";  
    $result=$db->query($query);                    d($result)
    if ($result)                                   r($result)
    {       
        ...
    }
}

You got two dr so we can safely assume that you got nothing to worry about; in both cases the values are assigned and read before being assigned again.

One might argue that you should make a single variable which's purpose is to indicate the result of a single query but that's up to you.