为什么$ header未定义？ [重复]

i am trying to send mail using php. i have removed the comment from the SMTP port and mail but Apache throws undefined variable $header in line no.5. what is wrong here?

<?php include "head.php";?>
<?php
$from= "smechailes@gmail.com";
$Headers = "";
$header .= "MIME-Version: 1.0 
";
$header .= "Content-type: text/html; Charset= iso-859-1 
";
$header .= "From: ".$from." 
";
$to = "setok321@gmail.com";
$subject = "test-mail";
$message ="<h1>hello</h1>";
$mail = mail($to, $subject, $message, $header);
echo (int)$mail;
?>
<?php include "foot.php";?>

</div>

Replace $Headers = ""; by $header = "";

Explanation:

PHP variables are case sensitive.

You have initialised variable $Headers and assuming it to be $header.

And concatenating to $header, which is undefined.

Either change $Headers to $header

OR

change

$header to $Headers at all places.

You are using the concatenating assignment operator when you use .=:

$header .= "MIME-Version: 1.0 
";

is equivalent to:

$header = $header . "MIME-Version: 1.0 
";

Meaning $header is used as a part of the assignment and should exist prior.

use below code, because on line 5 $header is not defined you are concatenate the string

<?php include "head.php";?>
<?php
$from= "smechailes@gmail.com";
//$Headers = "";
$header = "MIME-Version: 1.0 
";
$header .= "Content-type: text/html; Charset= iso-859-1 
";
$header .= "From: ".$from." 
";
$to = "setok321@gmail.com";
$subject = "test-mail";
$message ="<h1>hello</h1>";
$mail = mail($to, $subject, $message, $header);
echo (int)$mail;
?>
<?php include "foot.php";?>