Can you use Javascript to check whether a form value already exists within a MYSQL table prior to submitting the form

I have a PHP form which has dependent drop downs within it. Therefore I have the following javascript at the top of the page so it can refresh to form client side without submitting it.

------------------------------------------------
<script language="JavaScript">
function autoSubmit()      
{
    var formObject = document.forms[\'theForm\'];
    formObject.submit();
}

function process(){

document.theForm.action="close_pdp_process.php";
document.theForm.submit();
}

</script>
------------------------------

The autoSubmit() function refreshes the form based on the drop down selections without submitting it.

The process() function submits the form only when the user is happy and presses the ‘Save’ button which actions this function.

The process() function then passes the form values to a separate PHP page called “close_pdp_process.php” which uses $_POST superglobal variable to collect the data and perform the necessary actions.

The problem I have it that I would like to check whether the ‘userid’ and ‘reviewdate’ primary key values already exist within the MYSQL table prior to opening the “close_pdp_process.php” page, so the user has an alert and can correct the form first.  

I was thinking that a check within the process() function would be appropriate. Like

function process(){

/////CHECK WHETHER KEY (userid, reviewdate) EXISTS IN TABLE  HERE  - If it does show alert, If not continue //////
 
document.theForm.action="close_pdp_process.php";
document.theForm.submit();
}

I’m not sure how to do this as I would normally do the check using PHP. Can I mix PHP and Javascript in this way? Or is there a better solution?

Thanks