PHP表单空提交触发implode()函数无效参数报错求助

提交未填写任何字段的表单时，触发如下警告：

Warning: implode(): Invalid arguments passed in /var/www/xxxxx.com/contact.php on line 35

报错源于第35行对implode()函数的调用，相关PHP代码如下：

<?php 
$your_email ='email@gmail.com';// <<=== update to your email address

session_start();
$errors = '';
$name = '';
$lastname = '';
$phone = '';
$address = '';
$zipcode = '';
$city = '';
$state = '';
$interested_in = array();
$interested_in = '';
$visitor_email = '';
$user_message = '';
$ansday = '';
$hidansday='';
 
$qnday=rand(1,7);

$hidansday=$qnday+1;
if($hidansday==8) {$hidansday=1;}
$whatday=jddayofweek ( cal_to_jd(CAL_GREGORIAN, 1,$qnday, 2012) , 1 );

if(isset($_POST['submit']))
{
    $name = $_POST['name'];
    $lastname = $_POST['lastname'];
    $phone = $_POST['phone'];
    $address = $_POST['address'];
    $zipcode = $_POST['zipcode'];
    $city = $_POST['city'];
    $state = $_POST['state'];
    $interested_in = implode(",", $_POST['interested_in']);
    $visitor_email = $_POST['visitor_email'];
    $user_message = $_POST['message'];
    $hiddenansday= $_POST['hidansday'];
    $ansday = $_POST['ansday'];
    
    $dw1=trim(strtolower($ansday));
    $dw2=strtolower(trim(jddayofweek (cal_to_jd(CAL_GREGORIAN, 1,$hiddenansday, 2012) , 1 )));
                                
    
    ///------------Do Validations-------------
        
    $email = htmlspecialchars($_POST['visitor_email']);
    if (!preg_match("/([\w\-]+\@[\w\-]+\.[\w\-]+)/",$visitor_email))
    {
        $errors .= "\n E-mail address not valid ";
    }
    
    if ($dw1!==$dw2)
    {
        $errors .= "\n Please enter valid day for the Security Question";
    }
    
    if(empty($name))
    {
        $errors .= "\n First Name is a required field. ";   
    }
    if(empty($address))
    {
        $errors .= "\n Please enter your address. ";    
    }
    if(empty($zipcode))
    {
        $errors .= "\n Please enter your ZIP Code.  ";  
    }
    if(empty($city))
    {
        $errors .= "\n Please enter your City.  ";  
    }
    if(empty($state))
    {
        $errors .= "\n Please enter your State.  "; 
    }
    
    
    if( !preg_match("/^[0-9]{3}-[0-9]{3}-[0-9]{4}$/i", $phone) ) {
    $errors .= "\n Please enter a valid phone number "; 
    }
    
    if(empty($errors))
    {
        //send the email
        $to = $your_email;
        $subject="Web Form";
        $from = $visitor_email;
        $ip = isset($_SERVER['REMOTE_ADDR']) ? $_SERVER['REMOTE_ADDR'] : '';
        
        $body = "A user  $name submitted the contact form:
".
        
        "First Name: $name
".
        "Last Name: $lastname 
".
        "Phone: $phone 
".
        "Email: $visitor_email 
".
        "Address: $address 
".
        "Zip Code: $zipcode 
".
        "City: $city 
".
        "State: $state 
".
        "Interested In (Check all that apply): $interested_in 
".
        "Message: 
 ".
        "$user_message
".
        "IP: $ip
";    
        
        $headers = "From: $from \r\n";
        $headers .= "Reply-To: $visitor_email \r\n";
        
        mail($to, $subject, $body,$headers);
        
        header('Location: thankyou.php');
    }
}


?>

对应的HTML表单代码：

<p>
                        Service Interested In <small>(Check all that apply):</small> <br/>
                        <input name="interested_in[]" type="checkbox" id="interested_in1" value="Internet"/> Internet<br>
                        <input name="interested_in[]" type="checkbox" id="interested_in2" value="Telephone" /> Telephone<br>
                        <input name="interested_in[]" type="checkbox" id="interested_in3" value="Key System" /> Key Systems<br>
                    </p>

问题原因 #

当没有勾选任何interested_in复选框时，该字段不会被包含在$_POST数组中，直接调用implode(",", $_POST['interested_in'])会因为传入的参数不是数组而触发警告。此外，代码开头对$interested_in先赋值数组再覆盖为空字符串的操作完全多余。

解决方案 #

修改第35行的代码，先判断$_POST['interested_in']是否存在且为数组，再执行implode操作：

方案一：使用三元运算符简化代码

$interested_in = isset($_POST['interested_in']) && is_array($_POST['interested_in']) ? implode(",", $_POST['interested_in']) : '';

方案二：使用条件判断处理

$interested_in = '';
if(isset($_POST['interested_in']) && is_array($_POST['interested_in'])){
    $interested_in = implode(",", $_POST['interested_in']);
}

同时，建议删除代码开头的冗余赋值：

// 移除以下两行
$interested_in = array();
$interested_in = '';
// 替换为
$interested_in = '';

修改后，无论用户是否勾选复选框，$interested_in都会被赋值为合法的字符串，不会触发implode函数的警告。

内容的提问来源于stack exchange，提问作者Sarobar