ขอสอบถามเรื่อง การ Convert หน้าแรกของ PDF ให้เป็นไฟล์ JPG ครับ

<div class="row" align="center">
    <div align="center">
        <?php
        include_once ('include/connection.php');
        $dblink = connect_db();
        $txt_title = $_POST['txt_title'];
        $txt_mem_id = $_POST['txt_mem_id'];
        $txt_date = $_POST['txt_date'];
        $time = time()."-".rand(1,9999);


        if($_FILES["txt_file"]["name"] != ""){
            if($_FILES["txt_file"]["type"] == "application/pdf"){
                [glow=red]$pdfDirectory = "document/purchase/";
                $thumbDirectory = "images/purchase/pdfimage/";
                $filename = basename( $_FILES['txt_file']['name'], ".pdf");
                $filename = preg_replace("/[^A-Za-z0-9_-]/", "", $filename).".pdf";
                $thumb = basename($filename, ".pdf");[/glow]
            }else{ ?>
                <form action="view_news.php" method="post">
                    <div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel">
                        <div class="modal-dialog modal-sm" role="document">
                            <div class="modal-content">
                                <div class="modal-header">
                                    <h4 class="modal-title">ผิดพลาด..!</h4>
                                </div>
                                <div class="modal-body">
                                    <p><strong>รูปแบบไฟล์ : </strong><?php echo $_FILES["txt_file"]["name"] ?> ไม่ถูกต้อง</p>
                                    <p id="font-sunflower">สนับสนุนเฉพาะไฟล์ pdf เท่านั้น..! <i class="glyphicon glyphicon-remove"></i></p>
                                </div>
                                <div class="modal-footer">
                                    <button type="submit" name="close" class="btn btn-warning">ย้อนกับ</button>
                                </div>
                            </div>
                        </div>
                    </div>
                </form>
                <script>
                    $('#myModal').on('shown.bs.modal', function () {
                        $('#myInput').focus()
                    })
                </script>
            <?php }
            [glow=red]$pdfWithPath = $pdfDirectory.$filename;
            $thumb = $thumb.".jpg";
            exec("convert ".$pdfWithPath[0]." -colorspace RGB -resize 800 ".$thumb." ",$output, $return_var);
            copy($_FILES["txt_file"]["tmp_name"], $pdfDirectory.$filename);
            copy($_FILES["txt_file"]["tmp_name"], $thumbDirectory.$thumb);
[/glow]        }
        if($_FILES["txt_file"]["name"] != ""){
            $file = $filename;
        }else{
            $file = "";
        }
        $strSQL = "INSERT INTO tbl_purchase (pur_title, pur_file, pur_thumbnail, pur_date, mem_id)
VALUES ('$txt_title', '$file', '$thumb', '$txt_date', '$txt_mem_id')";
        $query = mysqli_query($dblink, $strSQL);
        if ($query) { ?>
            <form action="view_purchase.php" method="post">
                <div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel">
                    <div class="modal-dialog modal-sm" role="document">
                        <div class="modal-content">
                            <div class="modal-header">
                                <h4 class="modal-title">เพิ่มข่าวจัดซื้อจัดจ้างเรียบร้อยครับ..!</h4>
                            </div>
                            <div class="modal-body">
                                <p><strong>ข่าว : </strong><?php echo $txt_title ?> </p>
                                <p id="font-mint">ได้ทำการบันทึกเรียบร้อยครับ..! <i class="glyphicon glyphicon-ok"></i></p>
                            </div>
                            <div class="modal-footer">
                                <button type="submit" name="close" class="btn btn-primary">ย้อนกับ</button>
                            </div>
                        </div>
                    </div>
                </div>
            </form>
            <script>
                $('#myModal').on('shown.bs.modal', function () {
                    $('#myInput').focus()
                })
            </script>
        <?php }else{ ?>
            <form action="view_purchase.php" method="post">
                <div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel">
                    <div class="modal-dialog modal-sm" role="document">
                        <div class="modal-content">
                            <div class="modal-header">
                                <h4 class="modal-title">ผิดพลาด..!</h4>
                            </div>
                            <div class="modal-body">
                                <p><strong>ข่าวนี้ </strong>ไม่สามารถบันทึกข้อมูลได้ <i class="glyphicon glyphicon-remove"></i></p>
                                <p id="font-sunflower">กรุณาลองใหม่อีกครั้งครับ..! </p>
                            </div>
                            <div class="modal-footer">
                                <button type="submit" name="close" class="btn btn-warning">ย้อนกับ</button>
                            </div>
                        </div>
                    </div>
                </div>
            </form>
            <script>
                $('#myModal').on('shown.bs.modal', function () {
                    $('#myInput').focus()
                })
            </script>
        <?php } ?>
        <script src="js/jquery-1.10.1.min.js"></script>
        <script src="js/bootstrap.min.js"></script>
        <script>
            $(function() {
                $("#myModal").modal();
            });
        </script>