Strip_tags + htmlentities - No connection to the variable

Question

I have the following problem. I have a database connection, which uses a function called $FLD to bring the fields I want:

$FLD="idfca, dtafca, solfca";

After that I am publishing the $FLD for a variable:

var fld = "<? echo $FLD; ?>";

And within an Ajax document I created a loop for Scrool page, and tried to connect this variable with the following code:

$FLD = htmlspecialchars(strip_tags($_POST['fld']));

Only it is not connecting, and I used the same function to connect the loop variable in Javascript. How do I resolve?