Snippet of code, help?

[iagal]

If someone could just go through and comment these lines and explain what the lines are doing that would be beyond wonderful. I'm still learning the logic and what various symbols mean and do. Thank you SO much!

if (isset($HTTP_POST_VARS['county']) || isset($HTTP_POST_VARS['month']) || isset($HTT_POST_VARS['type'])) {
$query = $query . ' where ';
if (isset($HTTP_POST_VARS['county'])) {
$query = $query . 'county = \'' . $HTTP_POST_VARS['county'] . '\'';
if (isset($HTTP_POST_VARS['month']) || isset($HTTP_POST_VARS['type'])){
$query = $query . ' and ';
}
}
if (isset($HTTP_POST_VARS['month'])) {
$query = $query . 'month = \'' . $HTTP_POST_VARS['month'] . '\'';
if (isset($HTTP_POST_VARS['type'])) {
$query = $query . ' and ';
}
}
if (isset($HTTP_POST_VARS['type'])) {
$query = $query . 'type = \'' . $HTTP_POST_VARS['type'] . '\'';
}
}

[sijis]

what these lines of code is basically doing is building an sql query to get data from the db.

isset($HTTP_POST_VARS['county']) 
is checking to see whether the user clicked the checkbox, dropdown or whatever in the form that refers to 'County'. Using $HTTP_POST_VARS is the way to get variables from a form in PHP version 3.(something) and older.
The newest version of PHP use $_POST['county']

either way you can say that lines in several ways:
if($county){} // if you have global variables on in php.ini
if($_POST['couny']){} // newer version of php.
if($HTTP_POST_VARS['county']){} // older version of php
now you can add isset()
if(isset($county)){} 
if(isset($_POST['couny'])){}
if(isset($HTTP_POST_VARS['county'])){}

that all means basically the same. if the user did click on the county checkbox (or whatever in the form) then that condition (control structure) is true.

sources:
http://www.php.net/manual/en/language.variables.predefined.php ($_POST['var'] explanation)
http://www.php.net/manual/en/function.isset.php (what is isset(); ?)
http://www.php.net/manual/en/control-structures.php (if,else are control structures)