[PHP] PHP and MySQL ....dynamic query?

[Blueriver Networking Services]

Hello everyone. :)

I am attempting to construct a query with PHP and MySQL based on user input from a 
referring form page.  Here is an example of what I need this to do:

This will involve multiple variables, but I will just use 2 here:

if ($dig== "1") {

 $qual1 = "WHERE (track >= 0)";
}

if ($dig== "2") {

 $qual1 = "WHERE (track >= 0) AND (track <=100)";
}

if ($lot == "1") {

 $qual2 = "AND (acres >= 100)";
}

if ($lot == "2") {

 $qual2 = "AND (acres >= 500) AND (acres <= 650)";
}

// create SQL statement
$sql = "SELECT * FROM myTable $qual1 $qual2";

// execute SQL query and get result
$sql_result = mysql_query($sql,$connection)
 or die("Could not execute the query");

.....

You can see my problem here.  I want to be able to write parts of my sql statement 
based upon user input, and then execute it all at once.  How do I tie all of my $qual 
variables in so that this will read/execute it properly?

Thanks :)