如何从输入字段插入文本到PHP数组

我有一个带有输入字段的表单，当提交时通过SendGrid的WebAPI发送电子邮件。不把全部写出来，这里是重要的部分:

HTML:

<form action="sent.php" method="post">
<label>To:</label>
<input name="to" type="text" size="95" />
<label>Subject:</label>
<input name="subject" type="text" size="95" />
<label>Message:</label>
<textarea name="message" type="text" cols="71" rows="30"></textarea>
<input name="submit" type="submit" value="Send"/>
</form>

<?php
require 'SendGrid_loader.php';
print "I'm setting up variables here'n";
$user = 'XXXX';
$password = 'XXXX';
$to_email = array('XXXX@yahoo.com','XXXX@gmail.com','XXXXhotmail.com');

print "I'm creating a new SendGrid account'n";
$sendgrid = new SendGrid($user,$password);

$mail = new SendGrid'Mail();
$mail->setTos($to_email)
    ->setFrom('jacob@jacob.com')
    ->setSubject($_POST['subject'])
    ->setText($_POST['message'])
    ->setFromName('Jacob');
print "about to send email'n";
$result=$sendgrid->web->send($mail);
print_r($result);

我想做的是允许有人在"to:"输入字段中输入多个电子邮件地址，用逗号分隔(即to:"john@gmail.com, sue@gmail.com, gill@gmail.com")，并将它们输入PHP文件中的数组。我试图做到这一点，而不必创建多个to:输入字段。

有什么建议吗?

谢谢

!更新:

PHP的explosion()函数正常工作。这是我更新的工作代码:

HTML:

<form action="sent.php" method="post">
<label>To:</label>
<input name="to" type="text" size="95" />
<label>Subject:</label>
<input name="subject" type="text" size="95" />
<label>Message:</label>
<textarea name="message" type="text" cols="71" rows="30"></textarea>
<input name="submit" type="submit" value="Send"/>
</form>

<?php
require 'SendGrid_loader.php';
print "I'm setting up variables here'n";
$user = 'XXXXXXX';
$password = 'XXXXXXX';
$str = $_POST['to'];
$to_email = explode(",", $str);

print "I'm creating a new SendGrid account'n";
$sendgrid = new SendGrid($user,$password);

$mail = new SendGrid'Mail();
$mail->setTos($to_email)
    ->setFrom('jacob@jacob.com')
    ->setSubject($_POST['subject'])
    ->setText($_POST['message'])
    ->setFromName('Jacob');
print "about to send email'n";
$result=$sendgrid->web->send($mail);
print_r($result);