IF Clause in Left Join

Hi,

I really need some help.  I'm trying to use an IF statement in a multi-table 
left join.  I'm joining four tables (customers, contents, courses & 
locations).

In the contents table I have two fields, product_id and value_id that relate 
to different values in different tables depending upon the value in the 
product_type field.  If product_type='course' then the product_id and 
value_id relate to the courses and locations table, but if 
product_type='other' then the product_id and value_id relate to the product 
and prices table.

I have two options:

  1.  Match only the courses and locations where product_type='course' and 
NULL values if relating to the product and prices table.
  2.  Or change the query (and fields) if the product_type='other' to use 
the product and prices table.

I'm using Mysql 4.3.  Here's what I had that worked, but it did not take 
into account that the values in the product_id and value_id fields relate to 
different tables if the product_type != 'course'...

    $command = " CREATE TABLE $table[name] ";
    $command.= " SELECT $fields1, $fields2, $fields3 ";
    $command.= " FROM (($table[customers] AS c ";
    $command.= " LEFT JOIN $table[contents] AS o ON o.order_id = c.order_id) 
";
    $command.= " LEFT JOIN $table[courses] AS t ON o.product_id = t.id) ";
    $command.= " LEFT JOIN $table[locations] AS l ON o.value_id = l.id  ";

Here's my first attempt at employing the IF statement within the Mysql 
query:

    $command = " CREATE TABLE $table[name] ";
    $command.= " SELECT $fields1, $fields2, $fields3 ";
    $command.= " FROM ($table[customers] AS c ";
    $command.= " LEFT JOIN $table[contents] AS o ON o.order_id = c.order_id) 
";
    $command.= " IF((o.product_type='course'), (LEFT JOIN $table[courses] AS 
t ON o.product_id = t.id LEFT JOIN $table[locations] AS l ON o.value_id = 
l.id), NULL )";


Finally, if there is a way for me to override a column value using PHP 
during the execution of the mysql query, then I could accept a simpler 
solution.  If I can use PHP to check the value of product_type and then get 
the values of product_id and value_id from an array...  All I need to know 
is how to inject php values into my simple query.  Say:

SELECT field1, field2, field3, IF(o.product_type='course', 
$product_name[$product_id] ,NULL) AS product, field5 FROM table1 as a, 
table2 as o;

The key point here is that I must be able to output the final results of my 
query to a new table.

I'd be extremely grateful for any help anyone can provide.  Thanks.


Keith