Mysql INSERT查询不起作用，但其他工作正常

I have written this code to upload an item but the insert query is not working.

<?php
require("connect.php");

if(isset($_POST['submit'])) {
   move_uploaded_file($_FILES['file']['tmp_name'],"assets/img/portfolio/".$_FILES['file']['name']);
    $filename=$_FILES['file']['name'];
    if($filename == ''){
        echo "you didn't select any image!";
        exit;
    }
    $title = $_POST['title'];
    $desc = $_POST['desc'];
    $cat = $_POST['category'];

    echo "$title, <br>$filename<br>$desc<br>$cat";

    $sql = "INSERT INTO portfolio (Title, Img, Desc, Category) VALUES ('$title' ,'$filename' ,'$desc', '$cat')";

    if(mysqli_query($conn, $sql)){
        echo "<script>alert('You have Successfully added an item')</script>";
    }
    else{
        echo "<script>alert('Failed. Please try again')</script>";
    }
}

mysqli_close($conn);
?>

i have tried assigning a select query on $sql and it works. Please help

i found out that the word Desc which i had put as a row is a reserved keyword for Mysql. Thanks for your advice anyway.