Форма PHP генерирует 500 — Внутренняя ошибка сервера

У меня есть основная контактная форма, которая была добавлена ​​на мой сайт. Форма работает до тех пор, пока вы не нажмете «Отправить», и вы получите сообщение об ошибке:

500 — внутренняя ошибка сервера.
Возникла проблема с ресурсом, который вы ищете, и он не может быть отображен.

URL в адресной строке — это файл PHP, который заставляет его работать. Я проверил и перепроверил, убедившись, что адреса электронной почты и названия страниц верны, но я не вижу ничего плохого.

Вот код формы:

 <form method="post" id="captcha_form" name="captcha_form" action="mailform.php"><fieldset><ol>
<li id="ht"><label class="solo" for="htest">Leave this input blank</label>
<span  class="required">(required)</span>
<input type="text" class="soloinput" name="htest" id="htest" value="" />

</li><li><label class="solo" for="email">Email address:</label>
<span class="required">(required)</span>
<input type="text" class="soloinput" name="email" id="email" value="" />

</li><li><label class="solo" for="name">Name:</label><span class="required">(required)</span><input type="text" class="soloinput" name="name" id="name" value="" />

</li><li><label class="solo" for="subject">Subject:</label>
<span class="required">(required)</span> <select class="soloinput" name="subject" id="subject">
<option value=0> </option>
<option value=1>General Inquiries</option>
<option value=2>CAD Services</option>
<option value=3>Testing/Certification Services</option>
<option value=4>Inspection/Quality Audits</option>
<option value=5>Engineering</option>
<option value=6>DAPIA/IPIA/HUD</option>
<option value=7>Modular Services</option>
</select>

</li><li><label class="solo" for="message">Message:</label>
<span class="required">(required)</span>
<div class="soloinput"><textarea class="soloinput" name="message" id="message" ></textarea>
<br />
<input name="submit" id="submit" type="submit" value="Send" style="float: right;" /></div>
</li></ol>
</fieldset></form> &nbsp;<br />&nbsp;<br />&nbsp;

Вот «обратная сторона» PHP (mailform.php):

 <?php
$dontsendemail = 0;
$possiblespam = FALSE;
$strlenmessage = "";
$email = $_REQUEST['email'];
$message = $_REQUEST['message'];

$subject = array();
$subject[1] = "General Inquiries";
$subject[2] = "CAD Services";
$subject[3] = "Testing/Certification Services";
$subject[4] = "Inspection/Quality Audits";
$subject[5] = "Engineering";
$subject[6] = "DAPIA/IPIA/HUD";
$subject[7] = "Modular Services";
$subjectindex = $_REQUEST['subject'];
if ($subjectindex == 0 || !isset($_REQUEST['subject'])) die
("You did not choose a subject line.   Please hit your browser back button and try again.");
else $subject = $subject[$subjectindex];
$emailaddress = "[email protected]";

// Check human test input box
if(isset($_REQUEST["htest"]) && $_REQUEST["htest"] != "") die("Possible spam detected.
Please hit your browser back button and check your entries.");
// Check email address function
function checkemail($field) {
// checks proper syntax
if( !preg_match( "/^([a-zA-Z0-9])+([a-zA-Z0-9._-])*@([a-zA-Z0-9_-])+([a-zA-Z0-9._-]+)+$/",
$field))
{
die("Improper email address detected. Please hit your browser back button and enter a proper
email address.");
return 1;
}
}
// Check month function
function checkmonth($field,$checkrequiredob) {
if($checkrequiredob == 0) {
if(!preg_match("/^[\s]   {0,5}$|^(0|January|February|March|April|May|June|July|August|September|October|November|December)$/", $field)) die("Improper month of birth detected. Please hit your browser back button and try again.");
}
else   {if(!preg_match("/^(January|February|March|April|May|June|July|August|September|October|November|December)$/", $field)) die("Improper month of birth detected. Please hit your browser back button and try again.");
}
}
// Spamcheck function
function spamcheck($field) {
if(preg_match("/to:/i",$field) || preg_match("/cc:/i",$field) || preg_match("/\r/i",$field) || preg_match("/i\n/i",$field) || preg_match("/%0A/i",$field)){
$possiblespam = TRUE;
}else $possiblespam = FALSE;
if ($possiblespam) {
die("Possible spam attempt detected. If this is not the case, please edit the content of the contact form and try again.");
return 1;
}
}
// Spamcheck URL function
function spamcheckurl($field) {
if(preg_match("/to:/i",$field) || preg_match("/cc:/i",$field) || preg_match("/\r/i",$field) || preg_match("/\n/i",$field)){
$possiblespam = TRUE;
}else $possiblespam = FALSE;
if ($possiblespam) {
die("Possible spam attempt detected. If this is not the case, please edit the content of the contact form and try again.");
return 1;
}
}
function strlencheck($field,$minlength,$maxlength,$minresponse,$maxresponse) {
if (strlen($field) < $minlength){
die($minresponse);
return 1;
}
if (strlen($field) > $maxlength){
die($maxresponse);
return 1;
}
}
function checkphone($field,$checkrequirephone,$warning) {
if($checkrequirephone == 0) {
if(!preg_match("/^([\s]{0,10})$|^(((\+)?[1-9]{1,2})?([-\s\.])?((\(\d{1,4}\))|\d{1,4})(([-\s\.])?)(((\d{1,12}){1})|((\d{3,4}([-\s\.])?){2,3})){1}([\s]{0,10}))$/", $field)) die($warning);
}
else {
if(!preg_match("/^((\+)?[1-9]{1,2})?([-\s\.])?((\(\d{1,4}\))|\d{1,4})(([-\s\.])?)(((\d{1,12}){1})|((\d{3,4}([-\s\.])?){2,3})){1}([\s]{0,10})$/", $field)) die($warning);
}
}
function checkpriority($field,$checkrequirepriority) {
if($checkrequirepriority == 0) {
if(!preg_match("/^[\s]{0,10}$|^[\d]$/",$field)) die("Improper priority detected. Please hit your browser back button and try again.");
}
else {
if(!preg_match("/^[\d]$/",$field)) die("Improper priority detected. Please hit your browser back button and try again.");
}
}$name = $_REQUEST['name'];strlencheck($name,1,60,"You have not entered a proper name. Please hit your browser back button and check your name entry.","You have entered a name that is too long. Please hit your browser back button and check your name entry.");if ($dontsendemail == 0) $dontsendemail = spamcheck($_REQUEST["htest"]);
if ($dontsendemail == 0) $dontsendemail = spamcheck($name);

if ($dontsendemail == 0) $dontsendemail = checkemail($email);
if ($dontsendemail == 0) $dontsendemail = spamcheck($email);
if ($dontsendemail == 0) $dontsendemail = spamcheck($subject);
if ($dontsendemail == 0) $dontsendemail = strlencheck($email,10,255,"The email address field is too short. Please hit your browser back button and check your entry.<br />","The email address you have entered is too long. Please hit your browser back button and check your entry.");

if ($dontsendemail == 0) $dontsendemail = strlencheck($subject,1,255,"You did not choose a subject. Please hit your browser back button and check your entry.<br />","The subject you have entered is too long. Please hit your browser back button and check your entry.");

if ($dontsendemail == 0) $dontsendemail = strlencheck($message,10,10000,"The message field is too short. Please hit your browser back button and check your entry.<br />","Your message is limited to 10000 characters. Please hit your browser back button and shorten your message.");
if ($dontsendemail == 0) $dontsendemail = strlencheck($emailaddress,8,255,"You have not selected a recipient of your message. Please hit your browser back button and check your entry.<br />","Possible spam detected. Please hit your browser back button and choose a recipient for your email.");
if ($dontsendemail == 0) {
$message="";
$message.="Name: ".$name."\r\n";

$message=$message."\r\nMessage:\r\n".$_REQUEST['message'];
mail($emailaddress,"$subject",$message,"From: $email" ); include "emailsent.htm";
}

Затем пользователь должен увидеть страницу emailsent.htm с сообщением «Ваше сообщение было успешно отправлено. Кто-то скоро свяжется с вами!»

Я не вижу ничего, что может привести к ошибке файла на стороне сервера. Любая помощь приветствуется! Тем не менее, я новичок в кодировании PHP (хотя я могу редактировать), поэтому, пожалуйста, дайте много деталей!