How to use jQuery to get value from $_POST

How to use jQuery to get value from $_POST

Hi, I thought this would be straightforward but i can't seem to find out or work out how to access a variable posted via a form on the previous page. I have the following line in some jQuery code:
Code:
$( '#ddlSelectBox > OPTION[value=8]' ).attr('selected', true);


I basically want to check for the value sent via $_POST, then use it to make sure a dropdown is populated with the posted value (in the code above, where "value=8", i need to replace the 8 with whatever value has been posted. I have LiveHTTP headers and can see the value i want being posted as job_id=whatever - i just don't know how to get it

Anyone have any idea how to do this?