My system settings:
PHP4.3.6, Linux FreeBSD, Apache 1.3.2

The Linux already have install "sendmail" program and to set \usr\local\lib\php.ini, like below :
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; For Unix only. You may supply arguments as well (default: "sendmail -t -i").
sendmail_path = "/usr/sbin/sendmail -t -i"

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codes for sending mail:
note: demonstration PHP standard codes to send mail, real codes referer it.
------------------------------------------
PHP Code:
<?php
/* recipients */
$to  "mary@example.com" ", " // note the comma
$to .= "kelly@example.com";

/* subject */
$subject "Birthday Reminders for August";

/* message */
$message '
<html>
<head>
 <title>Birthday Reminders for August</title>
</head>
<body>
<p>Here are the birthdays upcoming in August!</p>
<table>
 <tr>
  <th>Person</th><th>Day</th><th>Month</th><th>Year</th>
 </tr>
 <tr>
  <td>Joe</td><td>3rd</td><td>August</td><td>1970</td>
 </tr>
 <tr>
  <td>Sally</td><td>17th</td><td>August</td><td>1973</td>
 </tr>
</table>
</body>
</html>
'
;

/* To send HTML mail, you can set the Content-type header. */
$headers  "MIME-Version: 1.0\r\n";
$headers .= "Content-type: text/html; charset=iso-8859-1\r\n";

/* additional headers */
$headers .= "To: Mary <mary@example.com>, Kelly <kelly@example.com>\r\n";
$headers .= "From: Birthday Reminder <birthday@example.com>\r\n";
$headers .= "Cc: [email]birthdayarchive@example.com[/email]\r\n";
$headers .= "Bcc: [email]birthdaycheck@example.com[/email]\r\n";

/* and now mail it */
mail($to$subject$message$headers);
?>