Php 基于用户输入SQL防止值为负数

我有一个名为tblitem的表，我现在想要的是，当我添加产品时，它将检查tblitem库存，如果它大于用户输入，它将显示一个警报，如数量不足

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在表中添加更新之前，可以使用用户输入的数量检查库存值

if($qty<=$row['stock']){   //Check the quantity value before inserting or updating
    $latestinv = $row['stock'] - $qty; 

    $updateq = mysqli_query($con, "UPDATE tblitem SET stock = '$latestinv' WHERE ItemName ='$itemname'");

    $query=mysqli_query($con,"INSERT INTO tblpurchase(ItemName,Quantity,LatestInv) VALUES('$itemname','$qty','$latestinv')");
} else {
    echo "<script>alert('Entered quantity is greate than available stock');</script>"; 
}

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希望这对你有帮助

酷。到目前为止，你在这方面做过什么尝试吗？另外，你的脚本易受SQL注入的影响，请考虑使用参数化查询。在PHP SIR中，我是新手，这只是一个项目，先生，但是谢谢你的建议：

<div class="form-row">
<div class="col-md-6 mb-10">
<label for="validationCustom03">Item</label>
 <select class="form-control custom-select" name="itemname" required>
<option value="">Select Item</option>
<?php
$rno=mt_rand(10000,99999);

$ret=mysqli_query($con,"select ItemName, stock from tblitem");
while($row=mysqli_fetch_array($ret))
{?> 

<option value="<?php echo $row['ItemName'];?>" <?php if($row['ItemName']=='stock'){echo "selected";} ?> ><?php echo $row['ItemName'];?></option>
<?php } ?>
</select>
<div class="invalid-feedback">Please select Item.</div>
</div>
</div>

if($qty<=$row['stock']){   //Check the quantity value before inserting or updating
    $latestinv = $row['stock'] - $qty; 

    $updateq = mysqli_query($con, "UPDATE tblitem SET stock = '$latestinv' WHERE ItemName ='$itemname'");

    $query=mysqli_query($con,"INSERT INTO tblpurchase(ItemName,Quantity,LatestInv) VALUES('$itemname','$qty','$latestinv')");
} else {
    echo "<script>alert('Entered quantity is greate than available stock');</script>"; 
}