if (isset($_POST['submit_enquiry'])) {
    $enquiry = new Enquiry();
    $enquiry->newEnquiry($_POST, $_FILES);
}

class Enquiry {

    private function _processFiles($_FILES, $caseID) {
        ...
    }

    private function _processForm($_POST) {
        ...
    }

    public function newEnquiry($_POST, $_FILES) {
        $caseID = $this->_processForm($_POST); // returns caseID
        $this->_processFiles($_FILES, $caseID);
    }
}

Array
(
    [file] => Array
        (
            [name] => Array
                (
                    [0] => Blue hills.jpg
                    [1] => Sunset.jpg
                    [2] =>
                )

            [type] => Array
                (
                    [0] => image/jpeg
                    [1] => image/jpeg
                    [2] =>
                )

            [tmp_name] => Array
                (
                    [0] => /tmp/phpwyLp86
                    [1] => /tmp/phpKJa4iw
                    [2] =>
                )

            [error] => Array
                (
                    [0] => 0
                    [1] => 0
                    [2] => 4
                )

            [size] => Array
                (
                    [0] => 28521
                    [1] => 71189
                    [2] => 0
                )

        )

)

foreach($_FILES as $file){
   $file['name']=$_POST['enquiryID'].'_'.$_POST['userID'].'_'.$file['name'];
}

// do upload