In securing simple email contact form

I have very basic PHP skills. As it stands, I'm trying to "reverse" engineer a contact form I found line that has some level security (I truthfully don't know how good it is). I think I got it, it works, but I just thought maybe some of you more experience in php can glance it over and see if it's actually still secure.

As I said, I reverse engineered it, so I don't know if I broke the security, but it does work just fine. As you can see it's pretty basic. I've been reading a lot and learning about what everything does, just worried about ordering and redundancy (want to avoid it if necessary)... basically anything I can do to clean it up as much as I can.

Code below:

<?php
            $to='my@mail.com';
            $sender=stripslashes($_POST['sender']);
            $email=stripslashes($_POST['email']);
            $subject=stripslashes($_POST['subject']);
            $message=stripslashes($_POST['message']);
            $body=  "Greetings,\n\n$message\n\n$sender" .
                    "\n" ;

            $displayForm=true;
            if ($_POST){
                $sender=($_POST['sender']);
                $email=($_POST['email']);
                $subject=($_POST['subject']);
                $message=($_POST['message']);
                $valid=eregi('^([0-9a-z]+[-._+&])*[0-9a-z]+@([-0-9a-z]+[.])+[a-z]{2,6}$',$email);
                $crack=eregi("(\r|\n)(to:|from:|cc:|bcc:)",$sender);
                $crack=eregi("(\r|\n)(to:|from:|cc:|bcc:)",$message);
                    if ($sender && $email && $message && $valid && !$crack){
                    if (mail($to,$subject,$body,'From: '.$email."\r\n")){
                $displayForm=fal开发者_如何转开发se;
        ?>
        <div>Your message has been sent successfully. Thank you for contacting us.</div>
        <?php
            echo '<p>'.htmlspecialchars($message).'</p>';
            }else {
        ?>
        <div>Your message could not be sent due to a system error. We apologize for any inconvenience.</div>
        <?php
            }
            }else if ($crack){
        ?>
        <div>Your message contained one or more anomalies, indicative of malicious content. Please consider revising your wicked ways.</div>
        <?php
            }else {
        ?>
        <div>You failed to complete a required field, or to provide a valid email address.</div>
        <?php
            }
                }
            if ($displayForm){
?>

<form action="./" method="post">
    <div class="contact-border"><input class="contact-textbox" type="text" name="sender" id="sender" tabindex="1" value="<?php echo htmlspecialchars($sender); ?>" /> <strong> Name</strong></div>

    <div class="contact-border"><input class="contact-textbox" type="text" name="email" id="email" tabindex="2" value="<?php echo htmlspecialchars($email); ?>" /> <strong> Email Address</strong></div>

    <div class="contact-border-noreq"><input class="contact-textbox-noreq" type="text" name="subject" id="subject" tabindex="3" value="<?php echo htmlspecialchars($subject); ?>" /> <strong> Subject (optional)</strong></div>

    <div class="contact-border"><textarea class="contact-textbox" name="message" id="message" cols="90" rows="15" tabindex="4" ><?php echo htmlspecialchars($message); ?></textarea></div>

    <div><input class="submit-button" type="submit" value="Submit" name="submit" /></div>
</form>
        <?php
            }
        ?>

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Your code looks alright, but might I suggest adding a CAPTCHA? I made the mistake of not putting one up on my site, and paid a hefty price in spam for a couple days.