send and bulk email php

<!----  send single email ------------------------->

$recipient = "info@srimaharajainn.com";

	// Mail subject
	$subject = "Good news! A reservation has been requested by $email";

	// Mail content
	$email_content = "Booking Enquiry from $fname
/// in mail content show like this
Arrival: $arrival
Departure: $departure

Phone : $phone
Message: $message";

	// Mail headers
	$email_headers = "From:info@srimaharajainn.com";

	// Main messages
	if (mail($recipient, $subject, $email_content, $email_headers))
		{
		echo "<h1>Email has been sent successfully!</h1>";
		}
	  else
		{
		echo "<p>Oops! Something went wrong and we couldn't send your reservation.</p>";
		}
  else
	{
	echo "<p>There was a problem with your submission, please try again.</p>";
	}
<!------- /// send single email ------------------->

<!----- bulk email ---------------------------->

<?php
    if(isset($_POST['frm_chk'])){
    $_Chk_box_value = $_POST['frm_chk'];
$subject1=$_POST['subject'];
$message=$_POST['message'];
    echo $to = implode(',',$_Chk_box_value);
    //echo 'Pass this to mail() funtion Recipients : '.$to;
    $recipient = $to;

    // Mail subject
    $subject = $subject1;

    // Mail content
    $email_content = "
Message : $message
";

    // Mail headers
    $email_headers = "From:bnarendra6036@gmail.com";

    // Main messages
    if (mail($recipient, $subject, $email_content, $email_headers))
        {
        echo "<h1>Email has been sent successfully!</h1>";
        }
      else
        {
        echo "<p>Oops! Something went wrong and we couldn't send your reservation.</p>";
        }

    }
    ?>
    <html>
    <body>
    <form name="form1" action="" method="post">
    <!--
    Dynamically You Can Print the Check Boxes here....
    -->

    <input type="checkbox" name="frm_chk[]" value="bnarendra6036@gmail.com" />bnarendra6036@gmail.com<br>
    <input type="checkbox" name="frm_chk[]" value="rajashekar9426@gmail.com" />rajashekar9426@gmail.com<br>
Subject : <input type="text" name="subject"><br><br>
Message : <textarea name="message" cols="20" rows="5"></textarea><br><br>
    <!--
    .....With Other Elements
    -->
    <input name="" type="submit">
    </form>
    </body>
    </html>
<!-------------- //bulk email ------------------------------->