Many a time, whilst scripting in PHP, I've hit a problem with data. Usually it's some silly fault I made in the sending or processing and other times I've forgotten to take out the remnants of an older piece of code which causes the data to act in an unwanted manner. When I hit these problems, the first thing I do is try and debug it by looking at the data before and after it has been passed. In the article I'd like to cover some of the techniques I find most useful.

I'm going to assume you have sufficient knowledge of PHP to understand what I'm doing in these examples. First of all ,let's look at a simple example such as a login. Here's the scenario:

A user (you) has input the username and password, which you know are correct, but the script isn't logging you in.

It's at this point I would take a look at the data being passed to make sure there are no changes. To do this I would,first, comment out any SQL queries being done. For example:

0:  $q = mysql_query("SELECT * FROM `user_table` WHERE `Username` = '$user' AND `Password` = '$password'");
1:  $query = mysql_fetch_assoc($q); 

2:  /* Becomes */

3:  //$q = mysql_query("SELECT * FROM `user_table` WHERE `Username` = '$user' AND `Password` = '$password'");
4:  //$query = mysql_fetch_assoc($q);

I would also do the same thing to any hashing processes on passwords and such.

Once I've eliminated the queries temporarily, I can start accessing the data. In this scenario I'm going to assume I've sent the data via post. It can therefore be accessed via $_POST. To access it I would use this code on the page which the login goes to:

0:   print_r($_POST);
1:   //I could also use this to make the formatting easier for me to read
2:   foreach($_POST as $post => $val)
3:   {
4:     echo($post."=>".$val);
5:     echo("<br />");
6:   }

1:  if(!$q) //This ($q) is the query we used earlier
2:  {
3:   /** error handler */
4:  }

5:  if(!mysql_num_rows($q))
6:  {
7:   /** No results returned */
8:  }
9:  else
10: {
11:   while($row = mysql_fetch_object($q))
12:   {
13:   /** $row->${...} */
14:   }
15:  mysql_free_result($q);
16: }