如何使用php捕获多个插入的空文本字段

I tried to catch the empty text value but it not work. May i have other way for catch empty value? this is my code. Please, help me. Sorry my bad English.

<?php
   error_reporting(0);
   include_once 'dbcon.php';
   if(isset($_POST['save_mul'])){       
   $total = $_POST['total'];


for($i=1; $i<=$total; $i++)
{
    $fn = $_POST["fname$i"];
    $ln = $_POST["lname$i"];

    if ($fn = "") {
        alert('Please enter the name');
    }else{
        $sql="INSERT INTO users(first_name,last_name) 
          VALUES('".$fn."','".$ln."')";
        $sql = $MySQLiconn->query($sql);
    }       

}?>

if ($fn = "") {

You have a assignment inside the expression. What you want is a check for an emtpy string like:

if ($fn == "") {

you are using alert which is a javascript function and you are not using comparison operator while checking empty text

 <?php
    if(isset($_POST['submit'])){
        echo 'submit entered'.'<br>';
        if($_POST['name'] == '')
        {
          echo 'enter name';
        }
        else {
        echo $_POST['name'];    
        }

    }

    ?>
    <form method="POST" action="">
      <input type="text" name="name" >
      <input type="submit" name="submit">
    </form>

It is also possible to use empty for comparison and echo for alert message in PHP inside the for loop

if (empty($fn)) {
    echo 'Please enter the name';
}else{
    $sql="INSERT INTO users(first_name,last_name) 
      VALUES('".$fn."','".$ln."')";
    $sql = $MySQLiconn->query($sql);
}