function DYNAMIC_CHECK(X)
    {
        $.ajax(
        {
            url:'<?php echo htmlspecialchars($_SERVER['PHP_SELF']); ?>',
            type:'POST',
            data:X,
        });
    }

if(isset($_POST['REGISTER']))
{
    $COUNT = mysqli_num_rows(mysqli_query($CON, "SELECT * FROM USER WHERE EMAIL='$EMAIL'"));

    if($COUNT == 1) { $EMAIL_ERROR='EMAIL ALREADY EXISTS'; }
}

echo $EMAIL_ERROR;

$.post( "test.php", $( this ).serialize(), function ( data ) {
 ...what to do with data (returns the result of test.php)...
});

$.ajax(
    {
        url:'<?php echo htmlspecialchars($_SERVER['PHP_SELF']); ?>',
        type:'POST',
        data:X,
        success: function(data) {
            alert(data);
        }
    });