500 - Internal server error youtube

Hi Guys,
im new to php.
Im getting a error on my thank_you.php page and getting this messsage.
Server Error
500 - Internal server error.
There is a problem with the resource you are looking for, and it cannot be displayed.
Its seems the php code making this error
the php code is

<?php

if($_POST){

    //an array of the fields that are allowed to be sent.

    $allowed_fields = array(

                        "name",

                        "company",

                        "tel",

                        "email",

                        "message");

                     
    $msg = "This is a message sent via Cam website<br /><br />";

    $msg .= '<table width="500" border="0">';

    //loop through all the posts, $key is name of field, $value is

 data entered

    foreach($_POST as $key => $value){

        //check this is an allowed field

        if(in_array($key,$allowed_fields)){

            $msg .= '<tr><td width="50%" style="background-color: 

"#CCCCCC" valign="top">'

.preg_replace("/([A-Z])/"," $1",$key).'</td>';

            $msg .= '<td style="background-color: "#DDDDDD" 

valign="top">'.nl2br($value).'</td></tr>';             
        }

    }

    $msg .= "</table>";

     
    $to = "richard@email.co.uk";

         
    $subject = "Contact Form";

     
    $headers  = 'MIME-Version: 1.0' ;

    $headers .= 'Content-type: text/html; charset=iso-8859-1';

     
    if(in_string("http://", strtolower($_POST['comment']), 1))

    {

        header('Location: notsentcoment.php');

    }

    else

    {

        mail($to, $subject, $msg, $headers);

    }     
}



function in_string($needle, $haystack, $insensitive = 0) {

    if ($insensitive) {

        return (false !== stristr($haystack, $needle)) ? true : false;     
    } else {

        return (false !== strpos($haystack, $needle))  ? true : false;     
    }

}

?>