Select from where? (Primary keys?)

[Chris Edie]

Hey, trying to design a somewhat dynamic search program in PHP but am stuck on a dynamic SELECT string ... basically I want it to look like this

$query = "SELECT * FROM `$table_name` WHERE PRIMARY KEY = $index_value"

so no matter what table I am searching, when $index value has a proper value it in, it will pull up the record.
The problem is that 'PRIMARY KEY' doesn't seem to work, is there a way to make it work, or do I have to have the name of the column that is the primary key?
Any suggestions?
Thanks,
-Chris